The power plant will create shock wave in the process of generating electricity in the nuclear explosion fusion power plant, whose propagation law of overpressure is an important foundation for researching the design of cavity wall. However, there is a certain lack of describing the peak overpressure of shock wave in the existing equations (for example, relative error is great). A new equation about the peak overpressure of shock wave, the time of positive pressure τ+ and the propagation coefficient of shock wave co are suggested by theoretical analysis in this article, from which we can get the propagation law of shock wave. The comparative results indicate that: the new equation is more suitable than the existing equations for solving the peak overpressure of shock wave in the power plant.