Various stair-climbing or cleaning mechanisms have been proposed or developed in the past decades. Most of them are big-sized, complex, and/or expensive, which hinder their practical application. In this study, a new stair-cleaning robot is proposed and developed. In the new robot, a pair of legs is used for stair climbing. Rotating around the main body, the legs of the robot drive the feet to execute revolving but translational motions, thereby allowing the robot to climb. The new robot is highly compact compared with most traditional stair-cleaning or climbing robots, because only one motor is utilized to drive the climbing mechanism and the legs can be retracted at both sides of the robot when it is not climbing. Its compact structure enables the new robot to move along the riser to do cleaning on the stairs, which is a prominent advantage over most similar robots. The cleaning device is designed similar to that of current ground cleaning robots. The sensing and control system is designed for successful stair climbing and avoidance of falls and collisions. Experiments show that the new robot has high success rates in climbing up or down common types of stairs, and thus can do cleaning on the related stairs.