Abstract: To meet the engineering application requirements of establishing nonlinear constitutive theory for fiber reinforced composites with different properties in tension and compression, this paper presented a generalized Hill yield criterion, which combines Drucker-Prager yield criterion considering the effect of hydrostatic pressure and Hill yield criterion suitable for anisotropic materials. Based on this theory, Sun and Chen's one-parameter plasticity model was extended to a unified nonlinear constitutive model incorporating the difference between the yield strength in tension and compression. The experimental results show that, the proposed model can be used to accurately predict the off-axis tensile and compressive nonlinear response for carbon fiber/epoxy (IM600/Q133) composites.