Abstract: In order to investigate the influence of particle size on the micro-macro crack evolution mechanism of yellow sandstone, uniaxial compression acoustic emission tests on yellow sandstone with different particle sizes were carried out systematically. Based on acoustic emission monitoring technology and focal mechanism inversion method, the evolution mechanism of microcracks in the process of rock deformation and failure was studied. Meanwhile, the macroscopic morphology of surface cracks and the microscopic morphology characteristics of specimen fracture were analyzed by scanning electron microscope and geometric fractal theory. The test results show that the size of the particle size and the type of cement can affect the strength of the rock. The laboratory test shows that the peak stress of the yellow sandstone decreases with the gradual increase of the particle size. By comparing the bI values (improved b values) and average acoustic emission energy of different sizes of yellow sandstone specimen in the deformation damage process, it was found that before the peak failure of all specimens, the average acoustic emission energy rate had phenomenon of "surge" and "plunge", and the bI values dropped to the minimum value when the sandstone specimen reached the peak failure, this phenomenon could be regarded as a precursor of rock instability and failure. The failure mode of microcracks in the rock changed from tension-oriented to shear-oriented as the mineral particle size increased. After failure, the fractal dimension of macroscopic fractures on rock surface decreased with the increase of rock particle size, that is, particle size has a certain control effect on the evolution process of macroscopic fractures on rock surface.

Key words: crack type, particle size, fractal dimension, acoustic emission energy ratio