To investigate the causes of plane gate jamming in the closing process, a combined approach of physical model experiments and CFD numerical simulations was employed to examine the hydrodynamic characteristics of plane gates during dynamic closure under free-flow conditions. The results show that under the upstream pressure head, plane gates equipped with upstream seals experience significant hydrodynamic thrust forces with minimal contribution from water-induced gravitational forces, rendering gate closure primarily dependent on self-weight. After a period of gate operation, excessive friction coefficient is identified as a significant cause of closing jamming. Under free-flow conditions, the permissible friction coefficient of the plane gate is mainly governed by the gate self-weight, the upstream-downstream head difference, and the effective area subjected to water thrust. Five countermeasures were proposed to facilitate the successful closure of the plane gate: reducing the friction coefficient of supporting materials, increasing counterweights, decreasing the upstream-downstream head difference, reducing the water thrust action area, and appropriately closing the radial gate.