The shearing of spent fuel assembly is a key step in the spent fuel reprocessing process. Due to the high radioactivity of spent fuel assembly, simulating assembly often combines brittle materials with stainless steel in shearing test research. In this study, Drucker-Prager model and Shear Damage model were combined to establish the shear constitutive model. As the typical representative of brittle materials, the mortar was chosen to be tested and the model was verified at the speed of 5, 20, 30, 40, and 50 mm/s. The experimental results matched with simulation well. Moreover, the shearing process of brittle materials were analyzed, including the elastic deflection, the plastic deflection and the breaking of the mortar. The results show that the shear force fluctuates in a certain range with shear speed increases from 5 mm/s to 50 mm/s, and the shear force first increases and then decreases with the increase of the tool clearance.