The present invention discloses a gravity balancing device for a rehabilitation robot arm, and belongs to the field of rehabilitation robots. The gravity balancing device includes a shoulder joint connecting member, an upper arm connecting member and a gravity balancing assembly; the shoulder joint connecting member and the upper arm connecting member are pivotally connected according to the human body bionic structure to simulate the rotational movement of the upper arm of the human body around the shoulder joint; the gravity balancing assembly includes a plurality of springs, wire ropes and guide pulleys, the wire ropes connect the springs to the shoulder joint connecting member and the upper arm connecting member, the spring tension is used to balance the gravity of the arm, and the guide pulleys are used to change the force directions of the wire ropes, thereby saving space and making the device structure more compact. Further, by locking different guide pulleys, the arm gravity can be still balanced by the spring tension after switching of the rehabilitation robot between the left and right hand training modes, thereby ensuring that the robot can still work normally after the training mode is switched.