Abstract: In order to improve the security performance of existing bridges, the reliability analysis and health detection for them are made. Bridge, as the evaluation unit, is divided into four combination items of underground part, support, top part and bridge surface. Each item consists of several subitems, and a system is built of three levels of subitem, item and unit, each of which is partitioned into four grades. In the grade evaluation of the combination items, the logic principle of the fault tree is introduced to build the relationship between the components reliability and structure reliability, and at the same fime the method of transmit tree evaluation is adopted. Besides, engineering fuzzy theory is used to analyze structure reliability, in which the evaluation grades of the parts of a bridge is decided by the percentage of the dangerous points, and the constitute attachment to the every grade is determined by the percentage of the dangerous constitutes of every part and finally the judgment matrix of bridge grade is built By the compound operation of the fuzzy matrix, the danger of the bridge can be judged and the evidence is provided for the reasonable decision of its maintenance and reinforcement.