Abstract: The mechanical analysis of bottom hole assembly is still in the phrase of static study, and some dynamic factors should be considered, i.e., calculating the bit boundary condition twice by taking the bit as an articulated support and an immovable support separately, based on the fact that the bending moment of bit is continuously changing, and then average the two results. The result of this method may be more reliable, as compared with the result obtained by taking the bit as just an articulated support or an immovable support.