Taking into account the influence of the internal micro cracks on the strength and deformation of concrete, a fourphase composite material model was proposed, in which it included mortar, aggregate, interface and random defects. Mesoscale unit in concrete was subjected to elasticplastic damage constitutive of increments form in strain space. With the simulation of mesh concrete crack growth by damage elimination, the damage processes of concrete under 3D uniaxial compression and the elastoplastic damage process of concrete under uniaxial tension with heterogeneous specimen were obtained. It was shown that the proposed fourphase model added the simulation of random defects, with which it was more appropriate to the actual situation in concrete forming. The percentage of random defects in volume was the key to determine the weakening degree of macrostrength of concrete. The cracks occlusion was obvious for the lower homogeneous uniaxial tension specimen and the fracture was clearer for higher homogeneous specimen. The simulation results was in good agreement with test results.