Abstract: The present invention relates to an online temperature monitoring system and method for a direct air-cooled condenser. A temperature sensor moving system is arranged on the surface of the heat exchange tube bundle of the direct air-cooled condenser, and a temperature sensor is installed on said temperature sensor moving system and moves on a plane parallel to the surface of the heat exchange tube bundle. The data collected by the temperature sensor is uploaded to a connected data processing system. Said system can collect, analyze, store and query data and give an alarm based on the predefined settings. Compared with the distribution of a plurality of monitoring cables in the prior art, the present invention realizes the scanning of a plurality of monitoring points, simplifies the cable distribution, avoids any blind monitoring area, and has flexibility.