A cross-sectional polishing method(taking a silicon as foil or taking a polyester as foil) and a bonded interface sectioning method were used to test the subsurface damage of RB-SiC (reaction bonded SiC)in rotary ultrasonic grinding(RUG) respectively. To determine the optimal test form, four kinds of subsurface damage evaluation indexes, namely average chipping layer depth, maximum chipping layer depth, average crack depth and maximum crack depth were used to analyze and compare the subsurface damages of RB-SiC in the RUG tested by the two methods mentioned above. The results show that the evaluation indexes from cross-sectional polishing method (the silicon as foil) are 3.30 μm, 6.59 μm, 8.64 μm, and 17.44 μm, those from the cross-sectional polishing method (the polyester as foil) are 5.71 μm, 14.33 μm, 15.36 μm, and 54.82 μm, and those from the bonded interface sectioning method are 9.19 μm, 19.45 μm, 13.04 μm, and 32.20 μm. It demonstrates that the cross-sectional polishing method (the silicon as foil) has the higher test accuracy, and the detection result is more in line with the actual situation. Finally, the paper summarizes subsurface damage characteristics of RB-SiC in the RUG.