Abstract: The investigation of the shear mechanical properties of infilled rock joints holds significant importance for the stability assessment of engineering rock formations. In this study, large-scale direct shear tests on infilled rock joint specimens were conducted, considering the influences of normal stress ( ), filled thickness (t), and joint roughness coefficient (JRC). The evolution patterns of shear failure characteristics, shear characteristics curves and peak shear strength were analyzed. Based on these findings, an empirical formula for the peak shear strength of infilled rock joint was established. The study reveals that the shear failure location of the infilled rock joint is primarily at the contact interface between the infill medium and the rock joint wall. The shear stress-shear displacement curve of infilled rock joint typically exhibits a shear stress peak when ≥600 kPa, t≤5 mm, and JRC≥6.36. At a infill ratio Δ of 1.12, the peak shear strength of the rock joint ( ) is close to that of the infill medium ( ), As Δ further increases, decreases below . The parameter W is introduced into the empirical formula for the peak shear strength of infilled rock joint to describe the interaction occurring at the contact surface between the infill medium and the rock joint wall, capturing the influence on the peak shear strength of the infilled rock joint. The calculated values from the empirical formula exhibit an average relative error δ of ≤9.01% compared to experimental values.