Abstract: The calculation expression of critical load in the analysis of bottom hole assembly("BHA") in the horizontal interval is derived on the basis of the energy method mechanism and optimization theory is used to solve the semiwave number(m) of deformation curve of BHA in the calculation of critical load which is the problem of difficult to determine directily.The method pressnted in this article has the characteristics of adaptability,it is applicable for the analysis on stability of BHA consisting of different pipe diameter and meterial in the horizontal interval.The application method of this method is illustrated by three examples.