Distortion of optical system makes focal length of advanced fire control dynamic performance test system different at different FOVs, and influences angle-measuring precision. A focal length calibration formula based on leastsquare method was used to obtain a unified focal length for all the FOVs and the influence of the distortion was reduced. For a specific fire control dynamic performance test system, its measurement result was 2.34″ by using the method, which improved angle precision from 20 to 5 arc-second. It is proved that the modified calibration formula can improve the performance specification of fire control dynamic performance test systems.