Based on the theory of fuzzy mathematics, a fuzzy comprehensive expert system is built for assessing the damage of the in-service bridges. In addition to the bridge damage factors of the conventional methods, bridge service life and loads change are considered in the expert system. The Analysis Hierarchy Process (AHP) is used to determine the weight of different index factors. The feasibility and accuracy of the assessment system is testified through the damage degree evaluation of a highway bridge in China, and then the testing results are compared with current codes in China. The assessment system could provide the basis for decision makers in determining the maintenance and strengthening schemes of in-service concrete bridges.