The invention provides a target surface stray light illumination measurement device and measurement method. The target surface stray light illumination measurement device includes a diffuse reflectionboard, a projection lens, a measurement instrument and a controller. The diffuse reflection board includes a frame-shaped base board and a diffusion reflection layer. The base board is used for covering a non-action area of a light valve target surface. The diffuse reflection layer is arranged on the base board. The reflection property of the diffuse reflection layer meets requirements of a cosine irradiator. Stray light cast on to the diffuse reflection board is reflected to reflection light beams through diffuse reflection of the diffuse reflection layer. The reflection light beams form animage zone through projection via the projection lens. The measurement instrument is used for measuring the illumination of the stray light of the non-action area projected to the imaging zone. The controller calculates the target surface stray light illumination according to the area and the illumination of the imaging zone. According to the invention, measurement of target surface stray light illumination on optical valves such as a DMD and the like can be realized.