Abstract: From micro level, concrete is idealized as a two-phase composite made of mortar matrix and aggregate inclusions, and random particle model is used to symbolize the meso structure of the concrete. The method of automatic mesh division for the random particle model is studied. A rigid body spring model (RBSM) which is adapted to analyze the microcrack is used to calculate the influence of pre-existed microcracks on the behavior of concrete material, and the behavior of concrete microcracks propagation is also studied in this paper.