Abstract: There are limitation and short comings by using the fuzzy judgment method to evaluate the roadbed stability in karst area at present. Based on the consideration of the fuzziness and hierarchy of the chief factors for the roadbed stability, a two-stage fuzzy synthesis judgment model for the roadbed stability in karst area is presented. The interval features of the subordinate degrees and the weight vectors on the factors as well as the nonlinear features of weighting operation of fuzzy matrices are discussed. By use of the nonlinear composition operator on fuzzy matrix, the interval mathematics theory and the relative superiority degree method to interval number matrix, the interval fuzzy judgment theory for roadbed stability in karst area is suggested. It can reduce the effect of subjectivity of judgment and reflect the effect of the bad factors on the judgment results. It is more rational and maneuverable. The calculated results and a comparison with other methods show that the proposed method is reliable and reasonable.