In order to enhance the military capability under the background of system of systems combat (SoS combat), according to the characteristics of system of systems defense, the analysis method of defense capability from system to platform to system of systems is discussed according to the force composition, and the mathematical model of system defense capability based on defense capability and capability weight is established, and the calculation results of system defense are given through a typical example. The simulation results show that the method has strong applicability and good operability.