Research on defocus detection is an important issue in optical manufacture and testing. Astigmatism method is widely applied in the defocus detection due to the advantages of simple operation and high sensitivity. However, the design of defocus detection system based on astigmatism method is under improvement. The sensitivity analysis of the measurement system based on the basic theory of astigmatism method is carried on considering factors such as system components machinability and spot size rationality. The suitable parameters are determined and geometric parameters of optical components are optimized by the optical design software Zemax. The MATLAB is used for ray tracing to calculate the error caused by facular energy heterogeneity and non-standard elliptic property. Calibration of image plane with zero signal has been completed and the accuracy of measurement has been improved.