This paper introduces the principle of four-quadrant detector and the various factors influencing its detection sensitivity. The influence of dead zone on detection sensitivity is analyzed in detail. The deviation of spot is simulated with Matlab software. The experimental system, including the circuit part and optical part, is designed. Light intensity of each quadrant of the detector is detected by the circuit part. When the width of the dead zone is 0.1 mm, the deviation of spot is measured for the light spot radii of 0.6 mm and 0.4 mm, respectively. The experimental and theoretical results show that the detection sensitivity increases with the size ratio of dead zone to spot increasing.