The invention relates to a mechanical lock of a hydraulic cylinder and belongs to the field of hydraulic elements. The mechanical lock of the hydraulic cylinder is used in various hydraulic driven leveling platforms or heavy machinery and comprises a cone-shaped piston and an elastic clamp head, wherein a cone-shaped sliding bearing is installed between the cone-shaped piston and the elastic clamp head, so that friction coefficient of a movement pair of the locking piston and the elastic clamp head is reduced, and the problem that a special high-pressure unlocking hydraulic system needs to be provided due to the fact that a conical-saucer type locking cylinder mechanical lock is large in radial dimension and high in unlocking pressure is solved. The mechanical lock of the hydraulic cylinder is large in locking force and small in size, improves the locking force by more than two times under the same size, is low in unlocking pressure which is generally lower than 16MPa, does not need a unique locking and unlocking high-pressure hydraulic system, simplifies design of the hydraulic system, reduces cost, improves reliability of the hydraulic system and can achieve locking at any position.