The work mechanism of one gas-steam type missile launching power system is complicated, so a mass of scaling-down tests are required to be accomplished in the engineering development of the launching power system. Therefore, the scaling-down similarity theory matched with this technology was desiderated to be researched. Through analyzing the physical model and mathematical model, the similarity constraint condition of the scaling-down model was provided, the scaling-down test methods of launch tube and different sections of upper and lower part of missile were established, he conflict between scaling-down and similarity constraint condition was solved, and this paper can provide reference for this launching power system and other underwater scaling-down test research.