Abstract: Well trajectory control is very important during directional well drilling and horizontal well drilling, especially in the horizontal sections of directional drilling and horizontal drilling. The influence of the geometrical attitude of well trajectory on the mechanical properties of BHA should be considered in these sections in order to control the well trajectory accurately which means right on the target. Through quantity calculation, the mechanical properties of BHA in specific geometric parameters of the well trajectory were analyzed in this paper, and a method for analyzing the transitional section has been put forward to guide design and construction of directional well drilling and horizontal well drilling in well trajectory control.