In order to evaluate energy parameters of propellant accurately and impersonally, thermodynamic parameters of different propellant formulations were computed by potential energy method, and the correlations between the thermodynamic parameters and expanding power capability of propellant combustion gas in barrel weapon were researched. The results show that the thermodynamic properties of propellant combustion gas products have an important effect on its expanding power, and the correlation between conventional energy parameters such as impetus, potential energy and its power capability is not identical. When propellant is used in weapons with larger gas expanding ratio, the power capability accords with potential energy, and the power capability accords with impetus when gas expanding ratio is small.