Based on the characteristic implicit scheme method suitable for the combined simulation of unpressurized flow and pressurized flow in the water conveyance system, considering the hydraulic characteristics of the inlet box culvert section, the box culvert transition section, the tunnel section and the outlet box culvert section, the corresponding characteristic boundary is derived. The transient flow analysis model of the current tunnel system is established to calculate and analyze the steady flow and unsteady flow of the system. The discharge capacity of the system under the typical water level combination and the hydraulic characteristics of the typical section are specially analyzed.