Abstract: The formation natural deflecting law and force are the basic geological foundation for the well trajectory control in directional wells and deflecting-protection in vertical wells. Starting with the basic principle of drilling string mechanics, this paper analyzed the interaction between the bit and formation, set up the mechanics model of the formation natural deflecting force, calculated the anisotropy index, deflecting coefficient of the formation and orientation drift factor in Qiaohekou area by using the real data of Qiaohekou in Jiangsu oilfield, and designed new drilling tool structure with the results.