Abstract: To address the shortcomings of the existing studies on the size effect of shear strength of rock joints, 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 angle are selected based on the progressively magnifying method, and the joint panels are produced by using the 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 devices. Finally, using the direct shear test system for large-scale joints independently developed by the anthors-research group, the laboratory direct shear tests under three normal stress conditions are carried out to obtain the shear mechanics and damage characteristics of the infilled joints. The size effects of the critical infill ratio and shear strength are analyzed. The 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 joints 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 the unfilled joints, while the size effects on the shear strength of the infilled joints are not obvious. Through the analysis of the size effect mechanism, without considering the influences of the average undulation angle of the joints, it is found that the average undulation height is the main factor affecting the size effects of the critical infill ratio and shear strength. The above research can lay an experimental foundation for establishing the size effect model for shear strength of the infilled joints.