The invention relates to a method for eliminating error due to sample installation during optical crystal ultra-precision processing subsurface damage detection, The method solves the problems of sample installation error during the optical crystal ultra-precision processing subsurface damage detection. The method comprises the following steps: 1) arranging the detected optical crystal; 2) adjusting an initial position of X-ray source; 3) forming an actual incident angle omega' between the X-ray source and the surface of the detected optical crystal, and fixing the actual incident angle omega'; 4) performing detection; 5) obtaining the characteristic spectral line information of diffraction generated by the X-ray source and the detection surface and the subsurface crystal structure under different actual incident angle omega' conditions; 6) calculating the sample installation angle error delta; and 7) correcting the actual incident angle omega' according to the obtained delta. The method can be used for eliminating error due to sample installation during optical crystal ultra-precision processing subsurface damage detection, which can be used for the field of surface damage detection of the optical crystal.