Abstract: By the method of piecewise ploytropic exponent, the whole recoil process was divided into four different stages and the actual thermodynamic characters of the gas in each stage were analyzed. Theoretical analysis and experiments indicate that there are some differences between the gas thermodynamic characters in counter-recoil mechanisms and those in ordinary pneumatic mechanisms. The characters of the gas in counter-recoil mechanisms are not always between the isothermal process and the adiabatic process. These can affect the performance of counter-recoil mechanisms. The calculation, considering internal friction and heat transfer, is more approximate to the facts than the known methods.