Abstract: To investigate the automatic-power ratio change mechanism of cone-ring traction drive, oil film mean velocity and spin velocity of inner and outer contact area under steady state and ratio change situation were analyzed respectively. With these velocity equations, the lubrication model of contact area was developed. Then, the traction equation along direction of ratio change (DoRA) was obtained. After that, the factors, as well as their influence on the traction along DoRA were studied. The research results figure out that, when shifting, because of the ratio-change-angle <em<θ</em<, the contact area will generate a side sliding speed along the generatrix of cone. So the traction along DoRA is produced, and it makes the conic ring move along DoRA. Then, automatic-power ratio change situation is achieved. The bigger ratio-change-angle <em<θ</em<, the smaller sliding to rolling ratio <em<s</em<, the higher velocity of input cone and bigger contact load is, the stronger traction along DoRA can be generated, which makes the ratio change process save more labour.