Abstract: To address the shortcomings of the existing studies on size effect of shear strength of rock joint, which do not consider the effects of differences in the filling degree. Firstly, four continuous-size natural joint morphologies with the similar average asperity height and average undulation angles are selected based on the progressively magnifying method, and joint panels are produced through 3D printing technology. Secondly, the infilled joints with five infill ratios are prepared by combining the rock-like material preparation method and the millimeter-level filling device. Finally, through the direct shear test system of large-scale joint independently developed by the research group, laboratory direct shear tests under three normal stress conditions are carried out to obtain the shear mechanics and damage characteristics of infilled joints. The size effect of critical infill ratio and shear strength are analyzed. Results show that as the infill ratio increases, the shear damage mode of the joint is mainly changed from "rock-rock" shear fracture to "rock-soil interface" sliding damage, and the normalized peak shear strength gradually decreases until it reaches the critical infill ratio and tends to be stable. As the size of the joint increases, the critical infill ratio gradually increases, ranging from 1.6 to 2.0. There is a more obvious positive size effect on the shear strength of unfilled joint, while the size effect on the shear strength of infilled joint is not obvious. Through the analysis of the size effect mechanism, without considering the influence of the average undulation angle of the joint, it is found that the average undulation height is the main factor affecting the size effect of critical infill ratio and shear strength. The above research can lay an experimental foundation for establishing the size effect model of shear strength of infilled joint.