In recent years, Project-Based Learning（PBL）model has been attached more importance to College English teaching in China, many researches focus on the application of PBL, but few touch on evaluation mechanism. This paper aims to analyze four core elements of evaluation mechanism: notion, subject, content, and method. Further discussions about the evaluation mechanism are based on teaching practice and a questionnaire. It is proved that scientific and effective evaluation mechanism can guide and motivate students’ project-based learning and ensure the quality of college English teaching.