Unidirectional carbon fiber composites were tested under multi-axial loading conditions including tensile/compression/shear loadings along and perpendicular to fiber directions. Compression dominated tests showed brittle fracture mode like local kicking/buckling, while tension dominated tests showed fracture mode like delamination and fiber breakage. Pure shear tests with displacement control showed material hardening and softening before total failure. Tsai-Wu failure criterion with Mises-Hencky interaction term was used as the yield criterion, and an associated flow rule (AFR) was used to describe the plastic flow of this material. The modified Tsai-Wu failure criterion was able to predict the failure of the composites under different loading conditions. A new material post-failure softening rule was proposed, where a new parameter ψ was introduced to represent different loading conditions of the composite. This model was implemented to ABAQUS/Explicit as a user material subroutine (VUMAT). Numerical simulations using finite element method well duplicated the elasticity and plasticity of this material. Failure features like delamination was simulated using cohesive surface feature. This model showed good capacity in predicting both failure initiations and fracture modes for the unidirectional fiber composites. This model has good application potentials to large-scale engineering problems.