The internal microcrack propagation and focal mechanism inversion of rock under load are helpful to understand the nonlinear fracture mechanical behavior of macroscopic crack propagation process. Therefore, uniaxial compression tests of yellow sandstone specimens with different particle sizes are carried out based on acoustic emission positioning technology. The test results show that acoustic emission positioning technology and moment tensor inversion can reflect the damage degree of rock more directly. In the test process, the microcracks of yellow sandstone specimens were mainly tensile cracks. With the increase of the mineral particle size of the rock, the proportion of shear microcracks in the specimen gradually increased, and the failure mode of the specimen showed a trend of transition from tensile to shear failure. Combined with the spatial distribution of acoustic emission source, it can be seen that the initial crack location and crack propagation direction are stochastic, but with the increase of load, the order degree of internal crack evolution keeps improving. Research results can provide some theoretical reference for excavation stability evaluation of underground rock mass engineering.