Studying the psychological state of historical subjects can help us understand and explain the role of human. Psychological changes of special groups in specific historical periods can provide a good way to analyze their behavioral logic. From the perspective of social psychology, this paper observes the changes of the middle-peasant class's mentality before and after the climax of the agricultural cooperative movement in Beijing, which can analyze and explain the behavioral changes of the middle-peasant class in the cooperative movement. Through the analysis of archives from some districts and counties in Beijing, we can roughly divide the mentality of the middle-peasant class into three aspects during the period of the agricultural cooperative movement. The first is the dissatisfaction caused by the conflict between realistic interests and the psychology of getting rich; the second is the disequilibrium caused by the horizontal comparison of different groups in the same period; and the third is the maladjustment caused by the difficulty of adapting to the production and life style of agricultural cooperatives. From these three mentalities, especially the interaction between policy decision-making and implementation process and farmers' social psychology can be a window into the gains and losses of various decisions in the agricultural cooperative movement.