Abstract: Poisson's ratio is one of the important parameters in the analysis of mechanical properties of materials and structures. In this paper, the nonlinear evolution behavior of major Poisson's ratio of 2D-C/SiC composites was studied. Firstly, based on Mini composite model and cross-ply laminate model, the thermal residual stress calculation model of 2D-C/SiC composites was established considering the transverse isotropic property of fiber. Secondly, the major Poisson's ratio calculation model of 2D-C/SiC composites was established by using shear-lag theory and classical laminate theory while considering the damage and thermal residual stress release mechanisms of the material. Finally, the strain response and Poisson's ratio evolution of the material were characterized by experiments, and the theoretical model was analyzed and verified. The results show that the internal thermal residual stress of 2D-C/SiC composites is large, and the release of thermal residual stress during tensile damage is responsible for the negative Poisson's ratio. The model prediction results of stress-strain curve and Poisson's ratio evolution curve are in good agreement with the tested curves, which indicates the accuracy and reasonability of the theoretical analysis model.