Abstract: In order to explore the fracture mechanism of horizontal fissure rock mass with different apertures, single-fissure rock mass specimens with different apertures are prepared by using cement mortar. The uniaxial loading tests are carried out on the prepared rock mass specimens based on the RMT-150B rock mechanics testing machine. The results show that under the same length of prefabricated fissure, with the decrease of fissure apertures, the initiation position of microcracks shifts from the middle of prefabricated fissure to the tip. To verify this phenomenon, a parallel bonded contact element based on PFC numerical simulation platform is used to establish a numerical model for fissure body for analysis. Under that condition, the boundary conditions of numerical calculation are consistent with the test environment, the crack initiation modes and fracture characteristics which are consistent with the test results are obtained. In order to further explore the initiation and propagation mechanism of micro-cracks in horizontally fissured rock mass, based on the yield characteristics of crack surface deformation, the evolution law of particle displacement field in the numerical model of micro-crack initiation and propagation is considered, a simplified model for fixed beam is proposed, and the mechanism of micro-crack initiation and propagation in horizontally fissured rock mass with different apertures is analyzed and explained.