Three dimensional structure of rice-duckweed-crab was founded in rice paddy field.The first floor was higher rice plants,which grow above water surface.The second floor was aquatic thallophyta thinazolla,which kept afloat on water surface.The third level was river crab under water.Three parts utilized the limited land resources as vector,utilized sufficiently water resources and sunlight energy at different levels,and formed a tridimensional open agrienltural system.