A “nuclear power plant robot” is introduced in this paper. It is developed by Shanghai University. Boss is chosen for testing obstacle surmounting capability of tracked mobile robot with 6-DOF. There are two swinging arms in the front of the robot and two in the rear, they can swing synchronously, respectively. The movement posture during the boss-climbing are discussed. As the poses of the arms change, the position of the robot's centroid will change, thus affecting the obstacle surmounting capability of the robot. The theoretical value of robot platform's maximal obstacle surmounting capability is obtained and compared with the test results. The effect of centroid position on obstacle surmounting capability is obtained, which provides theoretical basis for centroid position control in obstacle surmounting process. An improvement on the robot's structure is also presented in this paper.