Abstract: In mechanical analysis of the bottom hole assembly, the boundary condition at the bit has been taken as an articulated support, i.e., the bending moment is zero degree. A totally new concept is proposed in this paper, which looks the bit as an immovable support, i.e., the corner is zero. It points out that only when the bending moment reaches a certain value that the corner appears. The formula for calculating the maximum bending moment is also presented, and examples are given to compare the differences between the two methods.