Abstract: For performing the best obstacle-surmounting capability of four-track robot with two swing arms, obstaclesurmounting mechanism of the four-track robot with two swing arms and its capabilities of surmounting obstacles, including step and slope-climbing, and channel-crossing, are analyzed from the viewpoint of kinematics based on the obstaclesurmounting mechanism of the fixed two-track robot. Its motion mechanism and maximal obstacle-surmounting capability of step-climbing forward and backward are mainly analyzed. Taking CUMT-II coal mine exploring robot prototype as an example, three dimensional relationship diagrams of the step height, the elevation angle and arms' swing angle are drawn and the relation curves of slope gradients and the swing arms' swing angles are drawn. The theoretical value of maximal obstacle-surmounting capabilities of the prototype are obtained and compared with the test results. And the best obstaclesurmounting performance and the corresponding centroid and swing arm positions are deduced. This paper would provide theoretical basis for centroid position control in obstacle-surmounting process.