As the large angle deflection of a mirror is difficult to measure in an electromagnetic galvanometer system, a photoelectric mirror-angle-sensing device based on the quadrant photoelectric detector (PD) and LED was designed. First, the basic principle of mirror angle detection was analyzed, and based on the LED Lambertian radiation model, the mathematical model between the true value and the calculated value of the mirror deflection angle was established. Second, the system detection range was defined with the nonlinearity as the evaluation standard. Then, the influence of the detector's horizontal position, distance between the center of the mirror and the LED, and half-power angle of the LED on the detection range of the system were simulated and analyzed. Finally, an experimental platform was built for verification. The simulation and experimental results show that the established mathematical model is effective for mirror angle detection using Lambertian LED. With reduced distance between the detector and the LED, increased distance between the center of the mirror and the LED, and increased half-power angle of the LED, the angle detection range of the system can be improved.