The design of a four-quadrant detector for the semi-active laser seeker on the guided gun-launched projectile was studied. Several key parameters of the four-quadrant detector, including the photosensitive area, spectral responsivity, respond speed and noise equivalent power were discussed in the application. A bigger photosensitive area will be benefited to get a larger detection range, however, it will decrease the response speed and increase the positioning time. The spectral responsivity of the detector was chosen to be highly sensitive with the 1064nm wavelength laser, and its responsivities for the other wavelengths were relatively low. As the operation time is short, the response speed of the detector need to be increased as higher as possible, and many influencing factors were compromised in the design. A lower number of NEP was optimized for the better detecting capability. The orientation detection circuit and related positioning algorithm was developed to determine the yaw angle and pitch angle between the projectile and the target. Then the relative position errors, including the lateral and vertical deviation, were obtained from the four-quadrant detector. The influence of the size of facula with respect to the detecting sensitivity and effective measure ranges of the four-quadrant detector were analyzed. Finally, the sizes both of the facula and the four-quadrant detector were designed to ensure the most efficient use of the detecting sensitivity and effective measure ranges of the detector.