Abstract: The reliability of FEM strength reduction in slope stability analysis and its result evaluation are the key factors that influence the engineering application of this method. Based on the ultimate bearing capacity problem of foundation and the slope stability benchmark examples of Australian Computer Application Association(ACADS). This paper deals with the reliability, the slope failure criterion and result evaluation of FEM strength reduction in slope stability analysis. A result evaluation method based on the ratio of width and length of the plastic zone b/L was proposed. Results show that the FEM strength reduction method is effective in the condition of sufficient and reasonable mesh density. And there is no significant difference between different slope failure criterions in this condition, while the plastic strain zone connection criterion is the most objective criterion. The results show that the relevance are very high between the safety factor and the ratio of width and length of the plastic zone b/L.The smaller value of this ratio, the higher precision of the safety factor. And the relative error of safety factor is less than 5%when the ratio b/L is less than 0.05. If the slip mechanism and the slip face could be determined from the equivalent plastic strain distribution, the difference of safety factor from that of finer mesh grid is less than 5% and the accuracy of the FEM strength reduction method is sufficient.