In the finite element analysis of large-scale heat exchangers, the tightness analysis of tube-to-tubesheet joints of heat exchanger is classified into a highly nonlinear problem due to the existence of contact between tube and tubesheet, and there are a large number of tubes in the heat exchanger. These all make it difficult to analyze the tube-to-tubesheet joints in detail with the full model method. The traditional local model method simplifies the problem in a certain extent, but its boundary condition is different from the actual situation, which will result in an inaccurate result. In this paper, the sub model method is introduced into the tightness analysis of tube-to-tubesheet joints of the heat exchanger. Taking a U-tube heat exchanger as an example, the traditional local model method and the sub model method are used to analyze the tightness of tube-to-tubesheet joints respectively. The residual contact pressure of the seal ring on the contact surface of tube-to-tubesheet joints is taken as the criterion to evaluate the tightness of the joint. Variations of the residual contact pressure obtained by the two methods are comparatively studied. It is found that the traditional local model method is not conservative enough compared with the sub model method, and the sub model method can simulate more real boundary condition and obtain tightness conditions of the joint in different locations,which is a more effective analysis method. In addition, it is found that the choice of cutting boundary of the sub model has certain influence on the analysis results.