An artificial knee joint includes a joint seat, a support frame, a hydraulic device, a push rod, and a biasing member. The joint seat has a joint body and a rotatable member connected pivotally to the joint body and connected fixedly to a residual thigh. The support frame is connected rotatably to the joint body, and is connected fixedly to a prosthetic lower leg. The push rod is movable relative to the joint body between an extended position, where a free end of the rotatable member is spaced apart from the joint body to align the lower leg with the thigh, and a retracted position, where the free end of the rotatable member abuts against the joint body. The biasing member biases the push rod to the extended position.