In this paper, we propose a new method to solve decision-making problems under interval type-2 fuzzy sets (IT2FSs) environment. Firstly, an interval type-2 fuzzy sets cosine similarity model (IT2-Cosine) is defined to determine the correlation degree of each decision maker. Secondly, based on the proposed cosine similarity model, a new weight determination model is defined to determine the importance of decision makers. Then, Nie-Tan operator is used to extend the gain-loss function and construct the gain-loss matrix. Finally, the comprehensive gain-loss value of each scheme is calculated by the prospect theory, and the optimal scheme is selected according to the value. In addition, a case study and comparative analysis are made to show the superiority of the proposed method in determining the weight vector and depicting the psychological behavior.