An inner-diameter quenching device is provided with a workpiece holding assembly for holding a workpiece to be quenched, a spray cylinder, and a heating coil. The workpiece has a central axis. The spray cylinder is disposed at a first side of the workpiece holding assembly and driven by a lifting assembly to move along the central axis into an inner diameter of the workpiece. The heating coil is disposed at a second side of the workpiece holding assembly and located on and movable along the central axis. The heating coil and the spray cylinder are separated and movable in opposite directions along the same center axis, which can further reduce the size of the quenching device, making it suitable for quenching a small inner-diameter workpiece. Besides, the spray apertures are inclined, so that the cooling liquid can be sprayed out more easily to enhance the quenching effect.