In order to reflect the heterogeneity of concrete at mesoscopic level and simulate the mechanics response from the mesoscopic to macroscopical in the course of load, the random aggregate model according to the Monte Carlo method and Fuller Graded Formula is carried out based on the assumption that the concrete is a multi-phases composite material composed of matrix, and the material parameters are defined based on the test. In this paper, the Numerical Manifold Method introduced not only can correctly simulate stress, deformation and failure of concrete, but can simulate propagation of multi-cracks in the concrete, and failure plane growth can be searched by stress results automatically, so it can simulate the kind high discontinuous problem very well. On this basis, NMM is adopted to simulate the tests on concrete mechanical performance. The creation, propagation and fracture process of cracks in compression of concrete are present. The computational results are in good agreement with the experimental data.