Abstract: The whole world has paid more attention to the geological disposal of high-level radioactive waste. Granite is the candidate surrounding rock of high-level radioactive waste disposal project in China. It is very important to understand the strength and destructive characteristics of granite in the design and performance evaluation of disposal system. As aggregates of mineral grains, granite is a heterogeneous rock composed of minerals such as quartz, feldspar and biotite, and the macroscopic mechanical behaviors of rock is obviously affected by mineral grain size. Taking the granites in pre-selected Alxa area of China's high-level geological repository as example, the rock samples have obvious difference in grain size. The effect of grain size on granite mechanical properties is studied using uniaxial compression experiments with the combination of numerical simulation. Uniaxial compression experiments are conducted on MTS815 rock mechanics test system and numerical simulations are based on the particle flow code PFC^2D. In the process of numerical simulation, the actual spatial distribution of various components within the rock ISS obtained by image processing based on the surface image of the specimen. The particle flow model of granite based on the actual distributions is thereafter established. The model is used to simulate the uniaxial compression test of granite, and the reliability of the model is demonstrated by comparison with the experimental results. Experiment and simulation results show that the failure mode of Alxa granite is brittle and tensional. Most failure is parallel to the direction of compressive stress. Numerical simulation based on digital image processing method can be used to calculate the mechanical responses of rock materials with high efficiency and accuracy. Mineral grain size has little effect on elastic modulus and Poisson's ratio of the rock, but peak strength is obviously affected by the grain size. The main results are as follows:The strength of rock with fine grained and equigranular texture is high, and the strength of rock with coarse grained and inequigranular texture is low.