Thermal behaviors of combined weapon systems were analyzed by developed computational programs in this study. Also, temperature distributions of the materials of the system were measured according to the experimental conditions. Field tests that reflected the calculated thermal flow characteristics and the measured temperature distributions of the materials were prepared in a large environmental tester for the weapon systems. Boundary conditions of the analysis were the inlet and outlet conditions of the environmental tester and the low temperature limit of $-32^{circ}C$. The soaking time of the system, including a fuel tank and a battery in the environmental tester, was obtained by the programs developed in this study to carry out the experiment in the predicted conditions.