Abstract: During analyzing the drillstring dynamics in 3D borehole, initial bending of drillstring must to be dealed with. And the effects of position and deviation on contact force between the drillstring and borehole should be considered. Based on mechanical model of turning drillstring in 3D borehole, the affine coordinate system was introduced to analyze the deformation, contact force and friction torque. A new method was offered for study of drillstring dynamics.