By the minimum free energy method, the thermodynamic properties of the acetylene-ammonia propellant with different content of acetylene, and six propellants of liquid oxigen(LO_X) with acetylene-ammonia, rocket kerosene, CH₄, etc were calculated and analyzed. Results indicate that the acetylene-ammonia propellant with acetylene content of 25% is best in aspect of the specific impulse and security. When the ratio of the chamber pressure to nozzle exit pressure is 180:1, and ratio of the nozzle exit area to nozzle throat area is 35:1, the theoretical equilibrium specific impulse of LO_X/acetylene-ammonia is 372.63 s, which is higher than that of LO_X/rocket kerosene.