Based on electro-hydraulic power pulsed technology and the characteristic of plasma piston expansion theory, the mathematical model of plasma piston is established and the hydrodynamic equations of fluid in discharge channel are induced. The nondimensional differential equations of hydrodynamic characteristics in cylindrical sym-metry discharge channel are analyzed. The relationships between the discharge time and hydrodynamic characteristics of discharge channel, the electricity parameters of discharge circuit is found by solving the differential equations, which predict the pressure, radius, and ratio of expansion in discharge channel.