Borehole bending deflection distance is an important quality index in geological drilling project. Borehole bending and full-angle-half-distance method is introduced in this paper, which is most commonly used in borehole bending calculation model. By the describtion of the spatial position of each measuring point on borehole axis on basis of three-dimensional coordinate system and by means of three-dimensional geometric method, a formula about deflection distance in any depth of borehole is derived, and the influence degree of vertex angle and azimuth’s change to the deflection distance is discussed with a calculation example.