Based on the principle of the liquid-fueled rocket engine, a combustion model is proposed for the HVOF/HVAF system. The combustion gas components and temperature for different mixture fractions were analyzed. At lower oxygen content condition (under-stoichiometry), the combustion temperature is lower and the solid carbon content is higher. The whole fluent flow mode was proposed for the supersonic spray, which consists of the gas combustion, the accelerating process, the cooling process and the decelerating process in the atmosphere. The velocity and temperature distributions were calculated according to this model; the results fitted well with experiments. The combustion gas parameter distributions are almost identical in the barrel, but differ significantly in the atmosphere. For HVOF system, the under-expanded gas will expand in the atmosphere, while HVAF system exhibits an opposite behavior. At the gun exit, the combustion gas reaches supersonic velocities both for the HVOF and HVAF condition.