As one of the basic language skills, English writing has been playing essential roles in English teaching. In recent years, with the reform of the college entrance examinations, writing in English test has accordingly promoted its difficulty level. Exemplified by continuation, new test questions put forward novel requirements for the candidates, which calls for urgent promotion in writing competence from a student angle and timely update in teaching methodology from the teacher perspective so as to cultivate talents well-adapting to the development of the times. In this study, the researcher selected project-based learning (PBL) method, aiming to explore whether it could improve senior high school students’ English writing competence. Based on this research setting, the researcher adopted an action research design, invited 46 Senior Three students from a natural class as the research participants and carried out three rounds of PBL writing teaching across 15 weeks. By means of scale, interview, written talk and teaching log writing, the researcher collected four parts of data during the implementation, including scores, interview texts, written talk texts and teaching logs, which were later respectively used to represent students’ writing competence and teaching process evaluation. Between different teaching rounds, the researcher applied categorization analysis to classify and sort out the evaluation data of the teaching process, which further functioned as the main basis for teaching improvement of the following stage, promoting the process of action research. The researcher also used the data processing methods of significant difference test (paired t-test), categorization analysis and proportion calculation to explore the changes of writing competence brought by the implementation of PBL writing teaching, thus analyzing the influence of PBL on high school students’ writing competence. According to the results, PBL can effectively improve senior high school students’ English writing competence, and the main improvements lie in the content, vocabulary, language use and mechanics, yet no significant impact on the organization had been proved. At the end of the paper, the researcher proposed some suggestions for future research like using project process products as the basis of effectiveness evaluation, and applying PBL to students’ thinking quality training.