In order to explore the shear failure characteristics of fractured rock mass pre- and post-grouting, delivering essential theoretical support for the safe and stable functioning of large-scale rock engineering projects. A computational model considering the load transfer process among the hydraulic loading system, shear box, and rock joints was established, and a FDEM(finite-discrete element method) coupling algorithm for crack propagation and grouting support process that integrates time and size effects was developed. Numerical simulations of the shear fracture process of fractured rock masses were carried out at both laboratory and engineering scales. Main conclusions are these as follows. The rougher the joint, the more cracks are produced and the greater the extent of the damaged area; Significant changes in shear strength, damage distribution and damage pattern of joint specimens before and after grouting; Shear cracks are first generated at the top and bottom of the archway, which gradually run through the periphery of the roadway, expanding to a depth of 1.5 times the diameter of the roadway; When the size effect is taken into account, the number of cracks increases significantly and the distribution is more dense, and the depth of crack extension increases significantly, basically reaching 1.4 times the diameter of the roadway; Grouting can significantly reduce the deformation of the surrounding rock, control the development of cracks, and enhance the stability of the roadway.