The utility model relates to a rolling mill locking cylinder, in particular to a Morgan type closed rolling mill locking cylinder. The Morgan type closed rolling mill locking cylinder solves the problem that a Morgan type closed rolling mill locking cylinder is small in disc spring locking force. The Morgan type closed rolling mill locking cylinder comprises a cylinder body and a piston rod. A plurality of disc springs used as locking devices are arranged on the piston rod. The disc springs are arranged in a mode that two disc springs are overlapped and then folded. The locking force in a locking state is 57kN, and the locking force is about 1.6 times of an original locking force.