As a select of precision guidance,laser semi-active seeker has been widely used in the war today.By analyzing the energy of laser,the seeker can get the location of the target.Thus,the character of laser′s spot that receive by the seeker will influence its ability of tracking the target directly.According to the design specifications and the working principle of the single four-quadrant detector,the main parameters of the optical system is analyze in the paper.After this,the optical system is designed and optimized by ZEMAX.For comprehensive evaluation of optical system performance,the spot size and aberration are analyzed.The analysis shows that the optical system meets the design requirements and the quality of the obtained spot is better.