Abstract: The external steel-meshed mortar layer is a widely adopted technique for improving the aseismic performance of existing masonry structures. However, there are significant differences in the calculation methods of shear bearing capacity between the " Code for design of strengthening masonry structures " and the "Technical specification for aseismic strengthening of buildings ", making further research urgent. Using the finite-discrete element method, this study establishes numerical models of a total of 36 external steel-meshed mortar layer retrofitted brick walls, covering four mortar strength grades and various vertical compressive stresses at the wall top. By directly constraining the base nodes of the wall, sliding failure at the interface between the brick wall and the bottom beam is eliminated, and the accuracy of the modeling approach is verified against experimental results. Based on the shear-friction strength theory, a theoretical formula for the ultimate shear capacity of retrofitted walls is proposed. Its validity and reliability are demonstrated through the comparison with existing formulas in the literatures. In this formula, the shear capacity of the original brick wall is calculated using the shear strength data of masonry and the coefficient accounting for combined shearing and compression. The contributions of the mortar layer and the steel mesh are calibrated through the method of undetermined coefficients and real data from numerical simulations, ensuring both accuracy and practicability. The formula proposed provides a reference for improving the aseismic strengthening design theory of masonry structures.