Abstract This study aims to explore the relationship between psychological suzhi, self-esteem and peer relationship in middle school students,and the mediating effect of self-esteem on the relations between middle school students’ psychological sushi and peer relationship. The relationship between mental health and psychological suzhi is just like that between “surface and essence”, the psychological suzhi directly affects the level of mental health. Interpersonal relations is one of the standards of mental health, peer relationship is an important part of interpersonal relations, peer relationship is also one of the standards of mental health. Self-esteem is one of the standards of mental health too, and there is close relationship between self-esteem and interpersonal relations,self-esteem can effectively predict interpersonal relations. Then, there is direct evidence to show that self-esteem also effectively predicted peer relationship. The study about the relationship between psychological suzhi,self-esteem and peer relationship was not reported. So, we have three hypotheses.(1) Psychological suzhi could positively predict self-esteem and peer relationship (2) Self-esteem could positively predicts peer relationship (3) Self-esteem could mediate the relationship between psychological suzhi and peer relationship. Adopting a questionnaire survey method, a total of 938 middle school students (mean age = 14.8 years) from 2 schools of Chengdu city were randomly selected to take part in this study. All subjects were tested anonymously in the process. the questionnaire of middle school students’ psychological sushi scale(simplified version), the questionnaire of Rosenberg self-esteem scale and other-reported peer relationship have been acquired. In our study, middle school students' psychological suzhi questionnaire (simplified version) and self-esteem scale using self-reported to answer, peer relationship was measured by the way of other-reported. Given our research adopted two different methods, included self-reported and other-reported, the common method biases could be ignored. The research from the perspective of psychological suzhi, explores its influence on peer relationship, and use other-reported method to measure the peer relationship ,and they are also the innovation of this study. Results were as follows:(1)There were gender and schooling stage differences in peer relationship.Girls' peer relationship was significantly better than that of boys(t=-2.61, p<.01),Senior high school students' peer relationship was significantly better than that of junior high school students(t=-14.11,p<.001).(2)Middle school students’ psychological suzhi was positively correlated with self-esteem(r=.66,p<.001) and peer relationship(r=.20,p<.001), (3)Self-esteem was positively correlated with peer relationship (r=.18,p<.001).(4)Self-esteem could mediate the relationship between psychological suzhi and peer relationship,the mediating effect accounted for 27.7% of the total effect. First, we discussed gender and schooling stage differences in peer relationship. Girls' peer relationship was significantly better than that of boys and senior high school students' peer relationship was significantly better than that of junior high school students, the results are consistent with the conclusions of the previous studies. Second, the study revealed the relationship between psychological suzhi, self-esteem and peer relationship of middle school students. Self esteem is an important mediator variable between middle school students' psychological suzhi and peer relationship. psychological suzhi not only influences the peer relationship directly, but also has indirect effect on peer relationship through self-esteem, this was the most important result in our study. The results suggest that the school administrators and educators should pay attention to the important role of the students' psychological suzhi. To improve the level of psychological suzhi through psychological suzhi education,then improve the level of self-esteem, and finally establish a good peer relationship, the process and result have positive meaning and important value.