The invention relates to a capping method for planar curve paths of a free-flow spillway tunnel, belonging to the technical field of hydraulic engineering. The method utilizes a capping technology for the planar curve path of a free-flow spillway tunnel to reduce the roof height, and comprises the following steps: firstly calculating the Froude number before the planar curve path when the free-flow spillway tunnel has maximum discharge flow; then determining the copping efficient according to the ratio of the width of the free-flow spillway tunnel to the turning radius of the planer curve path, the Froude number and the corner parameter of the planer curve path; and finally back calculating the residual amplitude of tunnel ceiling according to a defined copping coefficient. The residual amplitude of capping is required to be less than that of the tunnel roof before the planar curve path when the free-flow spillway tunnel has maximum discharge flow. In order to ensure that the flow of water at straight segments after copping outlet is stable, the extending length of the copping segment extending to the straight segments is designed to be 2-4 times of the width of the free-flow spillway tunnel. After the planar curve path of the free-flow spillway tunnel is copped, concave-bank water constrain restricted by copping is controlled to below the copping, to avoid negative and rolling flow state caused by the way that current flings the arc tunnel ceiling, thereby ensuring stable current of the downstream straight segments and realizing safe flood discharge.