The invention provides a surgical robot and a mechanical arm of the surgical robot. The mechanical arm comprises a vertical movement joint, a gravity balance mechanism and a first transmission mechanism, wherein the vertical movement joint is provided with a connector for driving an external component to move along the vertical direction, and the gravity balance mechanism is connected with the connector through the first transmission mechanism for balancing the connector and the external component; the gravity balance mechanism comprises a balance structure and a first flexible body, the first transmission mechanism comprises a proportion wheel and a second flexible body, and the proportion wheel comprises an inner wheel and an outer wheel; the first flexible body is connected with the balance structure and the inner wheel, a first body of the second flexible body surrounds and is partially fixedly connected with the outer wheel, two ends of the first body are respectively connected with a first extension part and a second extension part of the second flexible body, the direction, surrounding the outer wheel, of the first body is contrary to the direction, connected with the inner wheel, of the first flexible body, the other end of the first extension part is connected with the upper part of the connector, and the other end of the second extension part is connected with the lower part of the connector, and the displacement variable quantities of the first extension part and the second extension part are equal when the connector moves.