Based on the practical requirement and feature parameters of high resolution detector, a continuous zoom optical system was designed. The Modulation Transfer Function (MTF) was close to diffraction limit. Some surfaces of front group, variable group and compensating group had very high tolerance sensitivity by analyzing. Therefore, the manufacture and assembly were hard to be achieved. In addition to using defocus to compensate the reduction of the performance of the system, the Z-axis group displacement of the front group was introduced to reduce the sensitivity of the first group, and the feasibility of mechanical structure of the first group was analyzed. An optimization of incident angle was put forward to reduce tolerance sensitivity. The result shows that the performance is enhanced effectively without increasing the costs.