The acceleration loads of projectile during launch process of two-stage light gas gun were studied by the developed computational fluid dynamics program. With the usage of LS-DYNA software, the diaphragm rupture pressure was calculated by finite element method. The influence of different waves rupture diaphragm on the maximum acceleration loads of projectile was analyzed, keeping the configurations of gun unchanged. It is found that the maximum acceleration loads can be reduced and the muzzle velocity objective can be achieved by choosing the ruptured wave appropriately and optimizing other operational parameters. Soft launch capability is provided for launching complex lifting configuration models up to hypervelocity.