作为资本市场的“守门人”，注册会计师在维护资本市场稳定、保障公众利益、提高财务信息质量和推动市场经济高效运转等方面发挥了至关重要的作用。近年来我国资本市场快速发展壮大，对注册会计师的需求越来越大，导致众多会计师事务所通过急速扩张规模、缩短审计周期、采用低价竞争等方式来抢占市场份额。随之而来的是会计师事务所的审计质量不断下降，审计失败事件频繁发生，会计师事务所的审计质量越来越不受社会信任。Z会计师事务所是我国本土一家中型规模会计师事务所，其通过并购地方性小型会计师事务所的方式大量设立分所，因此其服务范围和业务规模在短时间内快速扩大，但是该事务所的审计质量问题也频繁发生，近年来屡次受到监管部门的警示和处罚。故本文选取Z会计师事务所为研究对象，结合其对S公司审计失败的具体案例，并将PDCA循环理论贯穿于审计质量控制过程中，研究其在审计质量控制过程中出现的问题。

首先，本文梳理了有关审计质量、审计质量影响因素、审计质量控制、PDCA循环理论在审计领域的应用等理论和文献。然后具体介绍了审计双方Z会计师事务所和S公司的情况以及审计失败的过程。接下来结合PDCA循环理论来分析Z会计师事务所在审计S公司过程中存在的问题并分析其原因。最后根据分析结果提出PDCA各阶段相应的改进建议，并提出相应的保障措施。本文得出的研究结论为会计师事务所在结合PDCA循环理论进行审计质量控制时需要做好以下几点：第一，在P（计划）阶段要充分了解被审计单位的情况并准确评估自身的胜任能力，制定科学的审计计划，合理分配审计资源，并加强对分所业务承接活动的监督；第二，在D（执行）阶段要根据审计项目执行过程中识别出的风险动态地修正审计计划，对于函证程序要强化管理和控制，对于底稿要加强检查和监督；第三，在C（复核）阶段要将审计质量复核贯穿于审计的前中后全过程，落实多级复核制度以及质量控制部门的项目质量复核流程；第四，在A（处理）阶段建立以质量为导向的考核评价制度，总结审计经验，建立经验数据库；第五，外部保障方面，有关部门要加强外部监管力度和行业自律机制建设，并结合信息技术等手段提高审计质量。

本文的贡献在于：将PDCA循环理论框架运用于提升审计质量控制的研究中，并且选取具有审计质量控制问题的Z会计师事务所，结合其典型审计失败案例，总结出了具有针对性并可借鉴的会计师事务所审计质量控制优化措施，为我国注册会计师行业健康发展提供建议

Certified public accountants, as the "gatekeeper" of China's capital market, have played a crucial role in maintaining the stability of the capital market, safeguarding public interests, improving the quality of financial information, and promoting the efficient operation of the market economy. In the last few years, the Chinese capital market has seen tremendous growth, resulting in a heightened need for certified public accountants. Accounting firms have taken steps to increase their market share, such as enlarging their own operations, shortening audit cycles, minimizing audit processes, and embracing cost-effective competition. As a result, the audit standards of accounting firms have declined, resulting in a tarnished reputation, and the audit quality of accounting firms is becoming increasingly unreliable. Z Certified Public Accountants is a Chinese accounting company of moderate size, offering a broad selection of services. In the last five years, the company has had a string of audit failures and has been repeatedly cautioned and reprimanded by regulatory bodies such as the China Securities Regulatory Commission and the Ministry of Finance. Consequently, this article opts for Z accounting firm and merges its particular audit failure in S company to apply PDCA cycle theory to the audit quality control process, investigating the issues that come up during the audit quality control process.

To begin with, this article provides an overview of the theoretical foundations and literature concerning audit quality, the factors that affect audit quality, audit quality control, and the utilization of PDCA cycle theory in auditing. Subsequently, it elucidated the circumstances of both Z accounting firm and S company's auditors, as well as the repercussions of audit failure. Subsequently, we will apply the PDCA cycle theory to investigate the issues that Z accounting firm has encountered when auditing S company at different points and assess their causes. In conclusion, the proposed solutions for each stage of PDCA are based on the analysis of the reasons, and corresponding guarantee measures are put forward. This article suggests that accounting firms should take the following steps to ensure audit quality in accordance with the PDCA cycle theory: first, during the P (planning) stage, they should have a thorough understanding of the audited unit's situation and accurately assess their own capabilities; secondly, in the D (execution) stage, the audit plan should be adjusted in accordance with the risks identified during the audit project. For audit procedures such as letter verification, it is essential to reinforce execution control, and for the manuscript, inspection and supervision should be reinforced; Thirdly, in the C (review) stage, audit quality review should be conducted throughout the entire audit process, introducing a multi-level review system, and reinforcing the oversight of the headquarters over the branches; Fourthly, a quality-oriented assessment and evaluation system should be set up in the A (processing) stage, regularly summarizing audit experience, and creating an experience database; Fifthly, in terms of external protection, pertinent departments should reinforce external supervision and industry self-discipline mechanism construction, and raise the professional ethics level of employees.

This article applies the PDCA cycle theory framework to research on improving audit quality control, and selects a typical general branch development and expansion model and Z accounting firm, a medium-sized accounting firm in China with audit quality control problems, to summarize and provide targeted and reference based optimization plans for audit quality control of accounting firms, providing suggestions for the healthy development of China's certified public accountant industry.