The study of group psychology and behavior in the early 20th Century is in response to the arrival of mass society. Le Bon and his study of the popular mind has long been cited and discussed by scholars worldwide. This article attempts to restate the intellectual history concerning group mind and group behavior this period with three purposes: First, taking Wilfred Trotter, a forgotten pioneer in this field, as a core, the article aims at breaking the monopoly of academic discourse centering Le Bon. Second, this article is to allow thoughts of Trotter, Le Bon, Freud, and Tarde to intermingle, and to present the integration of a thought map of the group psychology and behavior in that period. Third, the article tries to develop necessary criticism in order to provide references for today’s group study. This article includes four parts: first, the arrival of Trotter and the Age of masses; Second, the formation of group psychology; Third, the occurrence of group behavior; Fourth, the consequences of group behavior and its criticism.