In this paper, the causes of the excessive dissolved oxygen in boiler feed water in the methanol unit of olefin plant at PetroChina Dushanzi Petrochemical Company are analyzed. The internal leakage of boiler feed water sampler, the compliance of boiler feed water sampling, the leakage at static sealing points of sampling system and the upstream water sources were systematically investigated. According to the investigation results, some plans were made including adjusting the dissolved oxygen of deaerator, standardizing the sampling process, carrying out the trial test of deoxidizer and checking the filler of the deaerator. These adjustment measures solved the problem of excessive dissolved oxygen in boiler feed water.