At present, most underwater robots have a single walking mode which cannot adapt to all sorts of varied and complex seabed terrain. To solve these problems, this paper has designed a new type of wheel/track transform mobile device. Through this device, the robot can selectively switch the walking mode according to different seabed conditions. The mode is switched from wheel mode to quadrilateral track mode, which effectively increases the ground contact area and reduces the ground specific pressure. In this paper, the wheel/track transform mobile device is dynamically simulated on both soft and hard terrain. The simulation results show that the mobile device has the ability to change the shape of the track. It can walk smoothly on the ground with soft and hard terrain, it effectively improves the walking performance of the robot.