The evaluation of alkali aggregate reaction in an existing subway tunnel is discussed in this paper.The petrographical examination of concrete thin sections was used to distinguish the alkali reactivity of the aggregates.Combined with alkali content analysis, it was deduced that part A of the tunnel had the potential for alkali-silica reaction.The typical form of alkali-silica gel was found in a broken surface of a concrete core,indicating the occurrence of ASR in this part of the subway tunnel.Some measures against ASR should be taken in order to continue use the existing subway.