Whether the properties of rocket-assisted engine of gun-launched missile in extra-service can satisfy design indexes is an urgent problem to be solved. Static firing test of rocket-assisted engine of gun-launched missile in the extra-service was carried out and the information of ex-factory acceptance was collected. The change trend and reason of internal ballistic performance of rocket-assisted engine were analyzed. The result shows that the properties of the rocket-assisted engine of gun-launched missile after several years of natural storage can satisfy the design indexes, but the maximal pressure is increscent to some extent. It is suggested that the charge of rocket-assisted engine of gun-launched missile in the extra-service be applied with computed tomography scanning before static firing test. This method can ensure safety in static firing test.