Designing of gun recoil mechanism needs calculating the propellant gas active coefficient accurately, to accurately calculate the propellant gas active coefficient, proposed a calculation method that based on CFD technology. The numerical method set up a 3D flow model by the software Fluent and Gambit of the gun without muzzle device, monitor the force of barrel, get the impulse of gun bore resultant force during after-effect period, and then calculate the propellant gas active coefficient according to the formula. In order to verify the correctness of numerical method, compared the result of numerical method with empirical, theoretical formula and experimental values. The conclusion that computation of propellant gas active coefficient based on CFD is accurately can be proved after comparative analysis.