This paper adopts the free energy minimigation technigue for The calculation of theoretical rocket performance under equilibrium flow condition. According to the analysis of optimization theory，the direct search method is employed in combination with computer program. A solid propellant ingredient is obtained for maximum specific impulse and maximum characteristic velocity. The numerical results obtained agree fairly well with the results obtained by graphical method for calculating the performance from the literature [5].It is userful for determining the optimum composition of solid propellant systems.