The invention discloses a nondestructive testing method for ultra-precision machining sub-surface damage of an optical crystal and relates to a nondestructive testing method for ultra-precision machining sub-surface damage, which aims to solve the problems in the prior art that detection is performed based on a certain specific sub-surface damage form or a sub-surface damage form in a certain tinysection area, the detection process often has destructiveness and the detection result cannot comprehensively and accurately reflect the sub-surface damage form of an optical crystal material in theactual machining process. The process comprises the following steps: 1, placing a detected optical crystal on a mobile worktable; 2, enabling X rays generated by an X-ray source to be parallel to thesurface of the detected optical crystal; 3, forming a certain included angle between the X ray and the crystal surface; 4, obtaining diffraction characteristic spectral line information during diffraction of the X ray and the crystal structure; and 5, repeating the steps 3 and 4, and completing nondestructive testing of the ultra-precision machining sub-surface damage of the optical crystal. The method disclosed by the invention is applied to the field of nondestructive test.