In the continuous firing process of the launcher, the firing rate is usually taken as a fixed value. As such, there exists an issue that low rate firing lacks shooting efficiency while in high rate firing cases the heat accumulates continuously and the temperature increases rapidly in the barrel of the launcher; consequently, this high temperature threatened the service life of the barrel and the safety of the propellant. In this paper, in contrast to the traditional firing method that has a fixed firing rate, a variable-rate firing method is proposed to solve this issue. Particle swarm optimization algorithm is exploited in this method to optimize the firing rate (cooling time) of the launcher with the objective that the launcher can conduct the entire shooting in a properly short time while without exceeding the safe temperature of the barrel. The temperature distribution of the barrel is obtained by establishing and solving the coupled model between the interior ballistics and the heat transfer process in the barrel. The optimization results show that the proposed firing method with variable rates can effectively improve the firing rate under the premise of ensuring the safety of the barrel temperature.