Abstract: The in-plane constitutive relationship of ceramic matrix composites holds significant reference value for the design and assessment of thermal structures. Experimentally and theoretically, the mechanical behaviors of 2D-C/SiC composites under axial compression, in-plane shear, and off-axis compression were investigated. Through monotonic compression and cyclic loading-unloading experiments, the stress-strain behaviors of 2D-C/SiC composites were studied, as well as the evolution of the unloading modulus and residual strain with respect to the unloading stress at different loading angles (0°, 15°, 30°, and 45°). Additionally, the in-plane shear strain response behavior and the evolution of the shear unloading modulus and residual strain with unloading stress were characterized. Theoretically, taking into account the orthogonal anisotropy, nonlinearity and microscopic damage mechanism of the material, the compression, shear and compression-shear coupling damage evolution behaviors of the material were described by applying the microscale mechanics and damage mechanics methods, and the characterization models for the in-plane shear and off-axis compression mechanical behavior were proposed. The compression-shear damage coupling coefficients and damage deactivation coefficients in the damage decoupling model were determined based on the damage evolution experimental data. The stress-strain behavior of the material under off-axis compression was simulated. The results reveal that the axial compression constitutive relationship of 2D-C/SiC composites is linear elasticity; in contrast, the in-plane shear constitutive relationship is nonlinear. Specifically, the unloading modulus decreases as the unloading stress increases, and the residual strain increases with the increase in unloading stress. Under off-axis compressive loading, the axial compressive stress inhibits the axial compressive damage of the material, while the in-plane shear stress plays a promotion, and the stress-strain response behavior of the material responses considerable complexity. Nevertheless, the consistency between the theoretical simulation results and the experimental curves demonstrates that the constitutive theoretical model based on damage decoupling can predict the off-axis compressive stress-strain behavior of 2D-C/SiC composites accurately.