Abstract: Well path prediction and trajectory control are directly affected by prediction of deflection capacity of PDM, single bent screw without stabilizer is widely used in CBM horizontal-butted well. using geometric algorithm in this paper and based on the theory of three-point deciding a circle, through analyzing similar algorithm about prediction of deflection capacity of PDM with single bent screw and two stabilizers when considering the influence of the bent position and well enlargement, combined with related theory of three-moment equations, the prediction of deflection capacity of PDM with single bent and without stabilizer is put forward, and verified through field practice, can beused in guiding prediction of deflection capacity of bottom hole assembly during directional drilling and trajectory extension in coal seam.