The invention relates to a unidirectional mechanical locking hydraulic cylinder and belongs to the field of a hydraulic cylinder. The unidirectional mechanical locking hydraulic cylinder comprises a hollow cylinder body and a piston rod; one end of the piston rod is positioned in the cylinder body; a wedged stop block is arranged at one end, positioned in the cylinder body, of the piston rod; thedirection of a wedge tip of the wedged stop block is arranged along the axis direction of the piston rod; a limiting stop block is arranged on the piston rod on one side of the wedge tip of the wedgedstop block; the limiting stop block is positioned in the cylinder body; a plurality of balls cooperatively used with the wedged stop block are arranged between the wedged stop block and the limitingstop block; an annular floating piston is arranged on the piston rod on one side of the bottom end of the wedged stop block; a push rod for pushing the positions of the balls is arranged on one side,near the balls, of the floating piston; the push rod performs reciprocating sliding together with the floating piston in a gap between the wedged stop block and the cylinder body; and the unidirectional mechanical locking hydraulic cylinder can realize unidirectional mechanical locking, can avoid displacement of the piston rod, is simple in structure and convenient to lock and can effectively reduce the slip risk of the hydraulic cylinder.