Based on the analysis of the first prototype of an assisting robotic arm (ARA) for minimally invasive thoracic surgery, an improved design is presented. A self-gravity compensation mechanism is developed to resist joint torques caused by the weight of the robot and to reduce the force exerted on the end-effector by the surgeon. With a motor built into the arm, the design of the ball-socket-joint is modified to widen the surgeon's field of view. The current pedal brake is replaced with an operating pushbutton to avoid maloperation. A control system for the improved ARA is developed. The experimental results show that the workspace and the locking torque can satisfy the basic requirements of a surgery.