In order to measure the gun slaving precision as an important target of fir control system, designed the gun slaving precision detecting system of double-theodolite. Firstly, on its system composition and working theory, proceed with math-modeling and algorithmic formulas deduction. Secondly, based on Windows XP system, the detection software programming is accomplished by VC++6.0. Finally, after system finished, progress experimental verification on certain vehicular howitzer. Finally, testing and verification for the design system is use out on some model of Vehicular howitzer.The experiment results verify that the design could be the effective measurement means of gun slaving precision target, which heightens the measurement precision and the working effect.