Abstract: Anionic surfactant (AS) in water sample was determined rapidly by flow injection analysis with an on-lineextraction functional module. Some problems in the determination, such as peak tailing, chloroform exudation at the outlet of extraction module and replacing extraction membrane frequently were solved by optimizing structure of the extraction functional module. The detection limit was 10.2 μg/L with the linear range of 50.0～1000.0 μg/L(r=0.9993). The precision (100 μg/L, n=7) and recovery were 2.5%RSD and 97.8%～112.6% respectively. The detection frequency was 18 samples/h. Compared with traditional methylene blue spectrophotometry, the method provides the features of simple operation, highefficiency and therefore can meet the requirements of the onsite batch water sample analysis.