Based on classical interior ballistic theory and flow model, the design of the mutiple pistons pulled by powder gas for submunition dispersion was investigated. The projection process of shrapnel gas multi piston was divided into three stages, and the interior trajectory mathematical model of gas multi piston projection considering gas leakage was established. Take a shrapnel projection structure as an example, the influence of gas leakage parameter on the projection process of shrapnel gas multi piston, and suggestion was put forward that area of gas leak of pistons should be strictly limited, to provide the reference for the propellant gas multi piston throwing structure engineering design.