The theoretical defects of conventional Janbu method are systematically analyzed in this study. On the basis of these theoretical analyses, three inequalities are brought up to rationalize the results of inter-slice shear force, the point of inter-slice normal force and the point of normal force on slip line. The new iteration formulas of rationalized Janbu method are deduced and the process of rationalized upper bound solution is given out. The results of the example given in the end show that the rationalized Janbu method can distinctly improve the results’ reasonableness, and can work out the corresponding reasonable thrust line. And the results of rationalized Janbu method under given safety factor can provide relatively reasonable slipping force and slipping moment for reliability design of reinforcing slope.