Abstract: The critical sliding surface is important for the reinforcement of geotechnical engineering in practice. The existing researches on the critical sliding surface are mostly based on the upper bound theorem, while the theorem and numerical solution method for the critical sliding surface based on the lower bound theorem or lower bound model are not available. In this study, the new critical sliding surface solution theorem is proposed based on the lower bound model, and the corresponding numerical solution method is also provided. The accuracy and reliability of the calculated results as well as the rationality and feasibility of its engineering applications are validated through the examples of an upright slope.