Some filamentous Cyanobacteria are potentially harmful to the safety of drinking water, but it is difficult to remove them in the process of water treatment. In some place of South China Pseudanabaena sp. and some cyanobacteria grow awfully in raw water ponds. Despite various methods had been used to control the growth of cyanobacteria in these ponds, no one were ever found to take effect long, thus it is necessary to search for new methods. As previous studies showed that barley straw was very useful in controlling the growth of some freshwater algae and they had no negative effects to local environment and ecosystem, the aim of present study is to find out whether or not barley straw could control endemic cyanobacteria, especially Pseudanabaena sp. in local raw water, if so, how long it may take. An application of 5 g·m^-3 barley straw were made in a sedimentation pond from March 2005 to May 2005, another sedimentation pond was used as a control pond without dosage of barley straw in the same time. It showed that Barley straw began to rot away in the second month after the treatment, then inhabitation of the growth of some algae were shown. The results suggested that the application of barley straw to suppress the growth of Pseudanabaena sp.was long-lasting and effective. Under local climatic conditions, it took one to two months for barley straw to take effects.