The invention relates to an evaluation method for subsurface damage of an optical crystal in ultra-precision machining and aims at solving the problems that the existing detection process of crystal surface damage always has destructiveness, and detection and evaluation results cannot comprehensively reflect structure characteristics of a subsurface damage layer of an optical crystal material in the actual machining process, so that deformation behavior of materials in the ultra-precision machining process of the optical crystal and the forming process of surface/subsurface damage cannot be comprehensively and accurately characterized. The evaluation method comprises the following steps: putting a sample on a worktable; adjusting the position of an X ray source; acquiring information of anX ray diffraction spectrum; starting an X ray detector to move along the circumference, and acquiring information of the X ray diffraction spectrums surfaces and subsurface damage layers of differentareas and different orientation of to-be-detected samples; evaluating the subsurface damage conditions of the to-be-detected samples according to the information of the diffraction spectrums. The evaluation method disclosed by the invention is suitable for subsurface damage detection and evaluation of crystals in the fields of information communication and aerospace.