According to the characteristics of jet flow in the curved bottom diffusion steep trough, the integration method along the water depth is adopted, and the influence of curvature, bottom slope and bottom friction on the flow is considered, the three-dimensional water flow motion equation system is simplified to the two-dimensional water depth average flow equation, and the characteristic equation system of the water depth average flow is derived. Combined with the engineering example of parabolic cylinder bottom diffusion steep trough engineering, the characteristic difference method is used to calculate the numerical calculation, and the calculation point distribution can be adjusted by interpolating time by time period, which can overcome the shortcomings of the irregular point distribution calculated by the characteristic line method, so that the calculation results are convenient and practical.