The electrical conductivity of combustion products of composite propellant containing Cs salt which used in magneto-hydro-dynamic (MHD) generator were calculated through Sahe equation and model of electrical conductivity. The basic thermodynamic parameters used in the model of electrical conductivity were calculated by applying thermodynamic equilibrium constant equation. Through theoretical calculation,the change of electrical conductivity along with the Cs salt content and aluminum content was obtained. The results show that for the given aluminum content,the optimal range of Cs salt content(mass percent) is 5%-10%; the higher combustion temperature and lower pressure of combustion chamber promote higher electrical conductivity.