In-situ inspection and lab study were combined to analyze the prestressed concrete girders of an existing railway bridge built in 1976 in North China. The reactive components in aggregates and the alkali content of concrete were investigated. Typical reaction product was found in site and was analyzed in lab. Residual expansion of concrete cores drilled from some girders was measured. From test results, it can be deducted that alkali silica reaction had affected the concrete girders and would continue to cause expansion in the future.