Abstract：Imbalance thrust force method (ITFM) has been used widely in the fields of industry，civil and railway in China. When the slip face is circle，the precision of ITFM is equivalent to that of rigorous method such as Morgenstern-Price method. But because the method unreasonably assums that the orientation of interslice resultant force is parallel to slice bases，for non-circle slips，the error of safety factor obtained from ITFM is up to several decades percent or more，which leads to hidden trouble inevitably. So，focusing on the non-circle slips，the error by ITFM in most kinds of conditions is analyzed thoroughly，and a few conditions with small error are obtained. For the analysis of slope stability，it is suggested that for non-circle slips，it is better not to use ITFM alone，but to adopt two or more methods，instead.
