By applying the ellipse theory to the configuration design of the tracked robot, a transformable tracked robot is designed whose tracks can be continuously tensioned while the track length unchanged. The position of the robot centre-ofgravity can be adjusted in a large range by swinging the swing-arm, so the robot has good obstacle-surmounting performance. To comprehensively understand the obstacle-surmounting performance of the robot, the processes of climbing the stair and crossing the gully are analyzed. Based on the movement mechanism of obstacle-surmounting, the key states of the robot are analyzed in terms of kinematics and dynamics. According to the actual situation, the theoretical values of the highest stair and the longest gully that the robot can surmount are obtained by taking the geometrical conditions, slip and stability as the constraint conditions. Then, a simulation platform is set up to test the stair-climbing and gully-crossing performances on the basis of theoretical values of the obstacles, further the prototype experiment is carried out, and the obstacle-surmounting ability is verified finally.