With the deepening of the academic research on fretting, fretting is found more and more in the mechanical transmission system. In this paper, a three-dimensional numerical model of fretting contact under oblique forces is proposed and used to study the fretting contact of homogeneous, heterogeneous smooth surfaces and rough surfaces. The conjugate gradient method is used to deal with the interaction of pressure and tangential force and the fast Fourier transform is used to accelerate the iterative process. The results show that during the process of oblique force loading, the contact area and slip zone increase, and the stick zone decreases. During unloading, the contact area decreases and the stick zone increases rapidly and then decreases. These conclusions are also applicable to oblique force fretting contact on rough surfaces. The determination method and influencing factors of the fretting region between built-up edge and planetary gear inner hole are put forward. The results show that the increase of meshing force, radius of planetary gear inner hole and the decrease of interference fit and friction coefficient have obvious influence on the increase of fretting region.