The disappearance of personal character and the development of emotions in the direction of the crowd are the salient features of the formation of the group. This means that the de-individualization of group psychology, the role of emotions being greater than the role of reason, and the loss of personal responsibility, are in sharp conflict with individual will. So, what is the way that mental groups produce collective psychology? How to stay awake in the unconscious factors of the group? What is the vital role of opinion leaders in this? The movie "Joker" fully demonstrates the process of polarization and answers these three questions. Under normal circumstances, people can perform the process of role determination, role performance and role construction in the interaction with others. Although a certain degree of role playing is required, there may even be phenomenon such as unclear role conflict or role interruption. That is to say, in the process of interacting with others, the individual can clarify what he feels, think, love, and hate in his heart, and provide a guide for subsequent cooperative behaviors. But in group life, individuals tend to lose their individuality and mistake group will for personal will. And the large and contagious will of the group can easily cause individuals to be emotionally coerced and become a part of the huge collective emotions. This is the inner psychological mechanism that Jokers can influence others and turn them into "Jokers" just like themselves after being transformed into a person who is completely opposite to the original self. They regarded the Joker who was the first to change as a god, surrounded him by his side and committed all kinds of crimes, enough to see the shadow of group polarization.