Abstract: To improve the prediction and control ability of well trajectory in very soft and very hard formations,the effect of rock mechanical property on well trajectory and related control measures has been studied.According to beam theory,the effect of rock mechanical property on bottom hole assembly(BHA)property was analyzed,on this basis,a bit steering model was developed considering rock mechanical properties;then the model was used to describe the effect of weight on bit,building force,rock drillability and bit anisotropy on well trajectory quantitatively.For Φ215.9 mm well bore,if the instant expending ratio of well bore diameter is less than 3%,bending house PDM is suitable,if expending ratio is more than 3%,bending sub matched with straight PDM has stronger steering ability.With rock drillability increasing from 1 to 10,well trajectory trending angle decreases from 6.00°to 1.00°.Bending sub matched with straight PDM is recommended for the drilling of soft formation and sidetracking.Bit with more side cutting ability matched with bending house PDM is more effective when drilling hard formations.