Abstract: With the problem of excessive dissolved oxygen in a nuclear power plant’s condensate, its cause was studied from three aspects of system air leakage flow, condenser heat transfer performance and vacuum pump capacity by the methods of theoretical analysis and experimental research. Finally, the oxygen content of condensate was reduced to around 3 ppb（1 ppb=10^-9） by blocking the air leak-in point and improving the performance of vacuum pumps. Established method can be used widely to solve the problems of excessive dissolved oxygen in nuclear power plant’s condensate.