The stress characteristic of mortar and aggregate in concrete were studied systematically based on the fully graded 3D random meso scale concrete model. The mechanism that mixing aggregate into mortar weaks the concrete strength was revealed. The correction coefficient between concrete macro compressive strength and mortar compressive strength was experimentally and numerically analyzed. The influence of aggregate volume fraction in concrete and concrete strength on the correction coefficient was discussed in detail. It is demonstrated that the strength of mortar cement material will be weaken by mixing normal aggregate and the strength of concrete is lower than that of mortar in the same proportioning condition. There is a significant error if the strength of concrete is directly used to replace the strength of mortar in meso scale model analysis. The strength should be corrected by the correction coefficient. The strength correction coefficient increases with the aggregate volume fraction, and makes no difference on the strength of concrete.