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