Abstract: To further elucidate the mechanism of acoustic emission (AE) response when a fractured rock is compressed, a new method for calculating the AE characteristics from the perspective of relative particle motion based on the moment tensor theory in seismology was proposed in discrete element simulation. The crack expansion process and mechanical-deformation characteristics were simulated by the particle flow code (PFC). Based on the new AE response simulation method, the spatial evolution of AE events, magnitude distribution and the orientation of the moment tensor were calculated. The results showed that the stress of the pre-cracked rock under uniaxial compressive conditions drops dramatically after the peak, the cracks expanded rapidly into a macroscopic crack, the acoustic emission events surge, and the brittle damage characteristics after the peak are obvious. The number of tensile and shear cracks obtained from PFC showed a similar growth trend temporally, while the number of tensile cracks was more than that of shear cracks. The acoustic emission magnitudes obtained based on seismology principally concentrated in the range of −6.9, −5.9, and the slope of the G-R statistical relationship (b value) was 1.81. The spatial orientation of the AE moment tensor coincided with the tensile-shear cracks: the moment tensor is mainly transverse in the wing crack region, with more tensile cracks than shear cracks; while in the main crack region, the moment tensor was heterogeneous, and the percentage of tensile and shear cracks is close to each other. The tensile and shear properties of microcracks were determined based on the T-k diagram and R-value theory of moment tensor. The trends of the percentage of tensile and shear cracks were basically consistent with those obtained by PFC, while the comparison revealed that using −20, 20 as the interval of tensile-shear crack was more reasonable than that suggested by Feignier et al. The findings provide beneficial reference for accurate analysis of the characteristics of rock cracking seismic sources and the identification of tensile shear properties.