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In order to study the effect and optimization feasibility of different pressure-reducing structures in the upper crown flow channel of medium and high head Francis turbines, taking the No. 4 unit of Hongshanzui First-Stage Hydropower Station as an example, four different depressurization structure models of upper crown channel established by UG were taken as the research object. Based on computational fluid dynamics（CFD）technology, shear stress transport (SST) turbulence model was used to simulate a total of 28 calculation conditions of four different upper crown drainage structures under seven flow rates. The research indicators were the flow distribution characteristics of leakage water, the lower side pressure of main shaft seal, the axial water thrust of the upper crown and the sealing performance of the comb ring. The results showed that there were some differences in the leakage water flow regime in different drainage and depressurization structures. In order to improve the sealing performance of the turbine main shaft, a combined drainage and depressurization structure with a runner pump can be adopted. Compared with other structures, this structure had significant effects on reducing the main shaft sealing pressure, the axial water thrust of upper crown and the leakage of upper crown clearance. Adjusting the geometric parameters of the pump blades or pump cover of the pressure-reducing structure of the runner pump can achieve the optimization purpose. In view of the leakage problem of the main shaft seal of the power station, the combined drainage and pressure reduction structure with runner pump can reduce the lower side pressure of the main shaft seal by about 15.98% on average and the axial water thrust of the upper crown by about 52.99% on average, which can greatly improve the operation efficiency of the power station. A runner pump was added on the basis of the traditional single drainage and pressure-reduction structure, which provided a reference for obtaining the best comprehensive benefit and its transformation and optimization of a medium-high head Francis turbine.