A structure of robot for polishing on large mould free-form surfaces was presented. The small polishing robot is a kind of wheeled mobile robot. It can accomplish machining while it is moving on large workpiece surfaces. Based on the structure of a small polishing robot, the coordinate system was established. According to the feature that nonholonomic constraints and holonomic constraints existed simultaneously, the constraint equation was set up. Accordingly, the dynamic model of a small polishing robot was established by using Lagrange method. And then, dynamics simulation was conducted, the curves of displacement, velocity, acceleration and driving torque were analyzed. The results of dynamics simulation indicated that the dynamic character of a small polishing robot is stable. This small polishing robot is applied and feasible.