Using a triangular fuzzy number to express the difference degree coefficient might lead to a large error when the connection components show a broad peak distribution characteristic. The optimal interval of the difference degree coefficient was determined, the difference degree coefficient was formulated with the trapezoidal fuzzy numbers, and an assessment method using the connection number based on trapezoidal fuzzy numbers was developed through analysis of the relationship between the difference degree coefficient and grade standard threshold. The method was applied to vulnerability assessment of the water resources system in the Chaohu Basin. The results show that the confidence interval of the vulnerability grade in the water resources system in the Chaohu Basin is ［2.89,3.44］ with a 75% confidence level. This method can not only obtain the confidence interval of the evaluation grade and effectively avoid the error induced by the broad peak of the difference degree coefficient, but also determine the priorities of vulnerability grades in different sub-regions using the expectation-variance method