Abstract: In order to calculate the vertical hydrodynamic force of the hydraulic gate during gate opening and closing, the vertical hydrodynamic force is divided into the down-suction force during gate opening and the up-lift force during gate closing. During gate opening, the fluid pressure energy is converted into kinetic energy at the gate bottom edge, while the flow is constant with the same opening, which results in the contraction of the fluid cross-section at the gate bottom edge. Once the supplementary of the air is insufficient, the fluid is separate from the bottom edge of the gate, and then the vacuum forms, which causes the down-suction force. During gate closing, the gate blocks the fluid and changes the flow direction, resulting in the uplift force. Then based on the proposed mechanism of the down-suction force and the up-lift force, this paper employed the fluid energy and momentum equations to deduce the calculation formula of the down-suction force and the up-lift force. Finally the numerical model verified by Ji-Shi-Xia Hydropower test data is utilized to perform a simulation analysis, showing that the simulation results are in good agreement with the calculated down-suction force and up-lift force by the proposed formula.