Change characteristic of dew days in Sanjiang Plain was studied and dew was in situ collected using the collectors of poplar sticks in soybean field and marsh land from May to Oct. 2003.Carex lasiocarpa is the dominant plant in marsh land.At the same time, temporal and spatial changes of dew content were analyzed,including seasonal change and vertical distribution between marsh land and farmland.Dewfall condenses mainly in summer and fall and the peak of dewfall appears in September.The number of dew days in the years of 1992～2003 ranged from 66 to 108,and the mean is 95.42.The dewfalls in July,August and September collected on the surface of soybean field were higher than those on that of marsh land,and the dewfalls in May,June and October were lower than those of marsh land.The dewfalls varied with the height and dewfall was the richest in canopy in marsh land,and vertical changes of dewfalls in soybean field were unconspicuous.The unit area actual dewfalls in marsh land and soybean field reached 20.68mm and 19.46mm in 2003,which accounted for 5.05% and 4.75% of the rainfalls of the same period,respectively.Dew plays an important role in keeping water balance of marsh ecosystem and farmland ecosystem,and may compensate the shortage of rainfall in growth season of crops.