YAG crystal is a typical hard and brittle material with Mohs hardness of 8.5 and insoluble in any acid or base at room temperature, so it is difficult to process. A step-by-step lapping process is proposed in order to explore lapping processing of YAG crystals in this paper. Based on the method of free abrasive lapping, the particle size of boron carbide (B4C) abrasive was reduced step by step during the lapping process, and abrasive W40, abrasive W28, abrasive W14 and abrasive W7 abrasives were selected for lapping in steps, and the particle size ranges of the four abrasive abrasives were: 40-28 μm, 28-20 μm, 14-10 μm, and 7-5 μm in order. The effect of the lapping pressure, the rotational speed of the lapping disc and pendulum, and the B4C mass fraction in the lapping solution on the lapping effect in each lapping step was investigated, and the optimal lapping parameters were derived; the depth of subsurface damage after lapping of YAG crystals was measured by cross-section microscopy method to determine the amount of subsequent polishing removal, and the relationship between the depth of subsurface damage hSSD and the surface roughness Ra after lapping was investigated. The study showed that the best lapping results were obtained for each lapping step when the lapping pressure was 44.54 kPa, the rotational speed of the lapping disc and pendulum was 60 r/min, and the B4C mass fraction in the lapping solution was 15%. The material removal rates of abrasive W40, abrasive W28, abrasive W14 and abrasive W7 lapping were 83.12, 57.32, 27.54 and 9.53 μm/min, respectively, and the surface roughness after lapping was 0.763, 0.489, 0.264, and 0.142 μm, respectively. The subsurface damage of the step-by-step lapping was 3.041 μm, which were measured by cross-section microscopy method, and needed to be removed by the subsequent polishing; The relationship between subsurface damage depth and surface roughness of YAG crystals after lapping with this lapping parameter is hSSD=41.46×Ra4/3. The study can provide guidance for the actual processing and production of YAG crystal components.