Abstract: The serious vibration occurred for the condensate downcomer of a certain 600MW air-cooled power generation unit with a single vertical standpipe. The analysis shows that the unstable flow of working medium in pipe, the larger head drop of condensate and the more concentrated energy release are the major internal factors of vibration. In addition, the giant impact force of the condensate drop performes the thrust force on the elbow of pipe and the vibration happens. The irrational pipeline structure and the flexible pipe system also cause the resonance. Pipeline structure need be reconstructed in order to eliminate the vibration after running for some time, but the reconstruction just increases the rigidity of pipe system, the inhere reasons that cause the vibration can not be eliminated. In order to eliminate the vibration, it is feasible to reduce the fluctuations of working medium and wallop of elbows through releasing energy and redesigning the waterfall pattern of the downcomer.