In order to monitor whether the light just passes the spatial filter pinhole center,an on-line detection method in compact space for large and complex laser system was designed. A prism is located close to the window of the spatial filter,so that the light from LED before CCD can be irradiated to the small hole. At the same time,the CCD can receive the image of reflected light,and thus on-line monitoring is achieved. The prism meets the required conditions,that the vertical distance between the prism cusp and the center of the hole is not less than 4 mm,and the angle of prism should be as small as possible so that the exit light is in parallel. Using the optical design software Zemax,the light path was simulated and the parameters of the prism were derived. The results show that the angle of the prism is 55°,and the right-angle side is respectively 10 mm and 14.3 mm. The experiment proves the feasibility and efficiency of the designed method.