Colorectal cancer is the third most common cancer in the world, which is a serious threat to human health. Endoscopy is the most effective way to screen colorectal cancer. However, it is very difficult to design a robot that can move in a complex colon. Soft robots can adapt to a variety of unstructured environments, and their interaction with humans is more secure. This paper presents a novel differential-drive caterpillar-based robot for colonoscopy, which is expanded by an airbag. Combining the advantages of the soft robot and the wheeled robot, the robot has both the flexibility of the former and the rapidity of the latter. The size of the robot in the contraction state is only ϕ28mm × 35mm and the length of traction module is only 28mm. The robot can be expanded to a diameter of 50mm and has a maximum speed of 5mm/s. The design concept of the robot and its traction module are described in detail. To ensure the stable movement of robot, the controlled output force of the robot caterpillar track is tested by experiments. In addition, the robot is placed in a curved bellow to move, and its motion validity is verified.