In order to reveal the effects of coarse aggregate of different size grades on the strain localization of mass concrete, we established a mesoscopic finite element model to simulate the damage and fracture of fully-graded aggregate mass concrete under the conditions of different mesoscopic material structures, so as to analyze the evolution process and distribution characteristics of the region of strain localization. In the simulation, the effects of coarse aggregate of different size grades on the strain localization were respectively brought to light by changing the coarse aggregate distribution of one individual size grade while keeping that of the other size grades unchanged. Numerical results show that most of the strain localization appear in the interfacial transition zone between the large coarse aggregates and the mortar, and the main distribution characteristics of the strain localization region is mainly controlled by the extra-large coarse aggregate. Moreover, the large and medium aggregates play important roles in the evolution process of the strain localization region directly or indirectly, whereas the small coarse aggregate has little effect. The research findings can provide a preliminary basis for reducing the simulation difficulties and computational demand by simplifying the mesostructure of mass concrete.