Short and medium span bridges are widely spread and extremely large in number． The safety and dura- bility of concrete beam bridges，as a major portion of all bridge types，are very important． A neutral axis position based monitoring method is proposed，and the corresponding index system and evaluation framework are constructed to evaluate the performance deterioration and safety reservation． Through theoretical and finite element analysis，the parameter characteristics and deterioration law of neutral axis index are investigated regarding various stress state of reinforced concrete beam at different prestressing levels． A structural health monitoring system has been installed and operated in real bridges to evaluate the overall performance of neutral axis index system． Neutral axis index is estimated and statistical analysis is carried out based on the observed response data． Ｒesults show that the neutral axis index system can effectively characterize the degradation and damage development of the structure，and the proposed method can benefit for bridge design and management，thus has potential value for practical populariza- tion．