Abstract: The calculation of the contact force on the drillstring in the hole is a complex nonlinear problem. This paper proposes a simple and direct method to tackle the problem. Firstly, a more precise model of 3D drillstring mechanics (which is called ABIS in short)is chosen. The contact force and the displacement of the drillstring on one node is computed. Then, the relationship between the contact forces on all other nodes and the displacement of this node is generalized. Then, the flexibility formulation is developed from them, describing the influence of the whole set of contact forces on the drillstring displacement, which is more explicit than that of the ABIS. Through that formulation, a new method of computing the contact force of drillstring is obtained and tested, which greatly increases the speed of iteration.