During cold ring rolling process,changing the sizes of forming rolls including driver roll and idle roll will lead to a change of amount of feed △h and contact areas between ring blank and forming rolls,thus a change of the shape and dimension of deformation zone located in the gap of forming rolls is found. It has a significant effect on metal flow and the forming quality of deformed ring. So the size effect of forming rolls on cold ring rolling was investigated by three-dimensional dynamic explicit FEM under ABAQUS environment. The obtained results thoroughly reveal the influence laws of the sizes of forming rolls on the average spread,fishtail coefficient,degree of inhomogeneous deformation and force and power parameters etc not only provide an important basis for design of the forming rolls and optimization of cold ring rolling process,but also reveal the plastic deformation mechanism of cold ring rolling.