Abstract: The vibration of condensate downcomer is one of the problems in the design and operation of the air cooling units, this problem affected normal operation and even brought serious consequences for the air cooling units(2?600MW) in Daba Power Plant phase III. For the purpose of improving the safety and economy of units, the downcomer model before and after the structure modification had been established by making use of finite element software ANSYS, the inherent frequency and vibration model were obtained through numerical simulation of vibration. The situations of vibration were analyzed, the cause of vibration was found. The solution for the vibration proof of the power plant design was proposed.