For missile ejecting power system with multi-gas generators， the normal way of solving the interior ballistic equations is assuming the design parameters， the number of generators and the ignition time of gas generators by experience， in order to make the interior ballistic parameters， such as the detached velocity， meet the index requirements. Once the setup of the gas generator parameters is unsuitable， the optimization solution is easily failed. A calculation method is proposed based on the interior ballistic target， such as missile detached velocity. The number， output flux and ignition time of multi-gas generators are calculated step by step. Finally the suitable propellant charge parameters and structure parameters of multi-gas generators are gained. The solution method was realized by programming using Matlab. The interior ballistic simulation for missile launching using different units of multi-gas generators was carried out. The parameters of multi-gas generators computed by this method can be used as the input of the later optimization of interior ballistic solution. The research can greatly support the initial interior ballistic design of missile launching power system.