Abstract: A safety device for a pre-filled syringe includes a device body, a protective cover, a release ring, an elastic member, and a holder. An inner wall of the device body of the safety device is provided with a snap-fitting portion and a bearing portion. The protective cover is movably provided in the device body, and the protective cover is provided with a hook claw capable of being fitted with the snap-fitting portion for positioning and a positioning base for limiting the protective cover from being separated from the device body. The release ring is movably provided in the device body. Two ends of the elastic member abut against the release ring and the protective cover respectively. The holder is connected to the device body, and the holder is provided with an inner holder claw.