The structure, function and benefits of the rice-duck agroecosystem were studied. The results showed that rice and ducks co-grew well all day during most of rice growing seasons with a density of 300-375 ducks per hectare paddy field and benefit each other. Rice ecosystem provided foods and habitat for ducks, on the other hand, ducks canght and fed on insects and weeds in the field, and their waste was also a kind of good organic fertilizer for the rice. The activities of ducks could help to decrease or control the harms of pests, diseases and weeds and improve rice-growing environment such as soil fertility, soil quality and so on. As a result, this kind of farming system could produce green food rice. The economic income of the rice-duck farming system was 808.5 yuan·hm^-2 higher than that of rice mono-farming system conducted by most farmers in China now. So the popularization and application of rice-duck ecosystem may have good ecological, economical and social effects.