Abstract: The constitutive model under complex stress is the theoretical basis for the design and performance evaluation of composite structures. This study focuses on the mechanical response of damageable and nonlinear ceramic matrix composite (CMC) laminates under combined loadings. Firstly, the expressions of off-axis damage compliance and damage stiffness matrices of single CMC layer as functions of fiber angle were represented on the basis of its on-axis orthotropic characteristics of damage evolution behavior and stress-strain response. In the following, the nonlinear strain response mechanisms of damaged laminated plate was considered and the deformation variable in terms of the mid-plane strain and curvature were divided into elastic and inelastic parts, respectively. Further, the classical laminate theory (CLT) was extended and improved, and the elastic deformation as well as the inelastic one of a laminated plate were formulated, respectively. Finally, the physical equation of the generalized internal force versus deformation of nonlinear CMC laminated plates was presented, which would supplement and enrich the mechanical characterization theory for nonlinear composite materials. In other words, it will serve as a new model and methodology for analyzing and predicting the deformation behavior of CMC laminates under complex external load. For orthogonal symmetric CMC laminates as a special case, the specific expressions of the relationship between mid-plane strain and internal force is presented. On this bases, the stress-strain behavior of a 2D-C/SiC composite laminate under on- and off-axis tension is simulated, and the reasonability and applicability of the constitutive model is verified.