A dynamic elastic-plastic microplane constitutive model for concrete based on M2 which is proposed by B.P. Bazant etc., is presented in this paper. The state of each microplane is characterized by normal deviatoric and volumetric strains and shear strain.The strain-stress relations for deviatoric and shear component is described as elastic-plastic model. A new concept: failure strain has been introduced. The stress shall be zero after the microplane stress reaches failure stain level. The reinforced effect of steel fiber on concrete is taken account in this paper. Finally 2 calculation examples have been adopted to verify the rationality and correctness of the proposed module.