The invention provides a U-shaped opening clamping ring lock for an actuator cylinder. The lock comprises an auto-kinetic moving piston installed in the actuator cylinder and driving a U-shaped opening clamping ring, a piston rod axially connected with the auto-kinetic moving piston and a locking ring used for locking up. The U-shaped clamping ring is a U-shaped opening clamping ring (5) with the conical surface of one end abutting against a shaft shoulder chamfer locking bevel (8) of the piston rod (6) and an annular clamping groove of the other end clamped with an annular boss at the tail of the auto-kinetic moving piston (2), the U-shaped opening clamping ring is driven to move by the auto-kinetic moving piston through a U-shaped annular groove arranged on the annular boss at the tail of the auto-kinetic moving piston, locking or unlocking of the U-shaped opening clamping ring lock is achieved, when the U-shaped opening clamping ring is located between a locking bevel (7) of the locking ring (3) and the shaft shoulder chamfer locking bevel of the piston rod, locking of the U-shaped opening clamping ring lock is achieved, and the U-shaped opening clamping ring lock is driven by the auto-kinetic moving piston to slide out along the locking bevel during unlocking. By means of the U-shaped opening clamping ring lock for the actuator cylinder, fundamental problems that in conventional mechanical lock structures heavy loading cannot be satisfied and an actuator cylinder body is scratched under a heavy-loading condition are solved.