The diagnostic function of educational evaluation refers to the effectiveness and ability of educational evaluation to judge the effectiveness, contradictions and problems of education. The evaluation process of science education is a process in which evaluators collect relevant information of the evaluated through observation, questionnaire and test. And made a rigorous analysis. It can make value judgments according to evaluation criteria, analyze or diagnose which parts or links in educational activities are well done and should be maintained and improved, and at the same time, it can point out where there are problems, find out the reasons, and then provide ways and measures for improvement according to these reasons. The process of education evaluation is the same as seeing a doctor. Only through scientific diagnosis can we "prescribe the right medicine". This role of educational evaluation makes it play a particularly important role in improving the quality of educational work.