Aiming at reducing the strong subjectivity caused by single value scores in seepage behavior assessment of concrete dams, the methods based on interval grading are suggested. The weight factors are introduced to represent the difference among evaluation levels of the experts. The degree of the disagreement on seepage behavior is described by an interval probability distribution function and the subjective weight is corrected based on the interval width. The Monte Carlo simulation method is then introduced into the comprehensive evaluation of the seepage behavior. The proposed method was applied to assess the seepage condition of the No. 7 section in a concrete dam and the probability of the assessment grade belonged to basic normal and above normal was 99.9%. The assessment result is basically normal and consistent with actual operation, showing the interval assessment method based on Monte Carlo simulation is feasible to evaluate the seepage behavior of concrete dams and it can also be applied to project assessment with high uncertainty in which single value grading is difficult to be applied.