A hopping rover that is actuated by internal forces, is an ideal mobility approach for the exploration of small solar system bodies. The internal actuators are used to provide torques to rotate the hopper and the contact force on the surface will drive the hopper to hop from the asteroid. In this paper, the 3-D dynamics model of a hopping rover on an irregular-shaped small body is established based on a polyhedron model. The hopper is modeled as a cube with three reaction wheel actuators along three orthogonal body axes. The hopping dynamics model is developed in the asteroid-fixed frame. Initialization of a hop and determination of sliding behavior on the surface are analyzed. Then the orbital-attitude couple motion after hopping is calculated. Based on a model for the asteroid Bennu, the influence of surface coefficients, friction factors and other factors on hopping motions are discussed. Finally, the possibility of lift off from the surface of small bodies by hopping is studied. The simulations show that smaller surface stiffness coefficient will increase the hopping distance but the sliding will decrease it. Besides, the non-uniform gravity on the irregular-shaped small body will change the hopping direction. The 3-D dynamics model of a hopping rover can be applied to the preliminary analysis of the hopping motion in an irregular-shaped asteroid. which provides a reference for future asteroid surface explorations.