机构投资者作为公司外部治理的组成部分，能够对大股东与中小股东利益冲突引起的第二类委托代理问题具有重要的监督作用。然而，机构投资者同样受到投资者有限注意力的约束，从而导致监督缺位的情况。为此本文构建了2008年-2021年全部A股上市公司的机构投资者分心指标，实证检验了机构投资者对大股东占款的外部监督作用。结果表明，机构投资者的分心程度与大股东占款呈正相关关系。此外，有更强监督动机的独立型的机构投资者分心时，会更显著地增加大股东的占款动机。并且，自身外部监督越强的公司，机构投资者分心时造成大股东占款增加的效应更明显；最后，研究发现内部资本市场使得国有企业在机构投资者分心时大股东占款现象较民企更为突出。本文拓宽了大股东“掏空”动机影响领域的研究，验证了机构投资者的有限注意对资本市场的影响以及机构投资者的公司外部治理作用。

Institutional investors, as a pivotal constituent of external corporate governance, assume a paramount supervisory function in mitigating the second category of agency issues engendered by conflicts of interest between dominant and minority shareholders. However, institutional investors are also constrained by the limited attention of investors, leading to a lack of supervision. To address this issue, this paper constructs a distraction index for institutional investors in all A-share listed companies from 2008 to 2021 and empirically examines the external monitoring role of institutional investors on the expropriation behavior of major shareholders. The results indicate a positive correlation between the distraction level of institutional investors and the expropriation behavior of major shareholders. Furthermore, independent institutional investors with stronger monitoring motives significantly increase the incentive for major shareholders' expropriation when they are distracted. Moreover, companies with stronger external monitoring exhibit a more pronounced effect of increased expropriation by major shareholders when institutional investors are distracted. Finally, the study finds that the internal capital market makes state-owned enterprises more susceptible to major shareholders' expropriation when institutional investors are distracted compared to private enterprises. This paper broadens the research on the influence of the "tunneling" motivation of major shareholders, verifies the impact of institutional investors' limited attention on the capital market, and explores the external governance role of institutional investors.