Abstract: This paper studies the structural effect, energy and nutrient flows and economic benefits of "high bed and low ditch" farmland ecosystem in flood plain areas of the Pearl River Delta. The system can lower underground water level, retain soil deposits, reduce soil erosion and slow down the decomposition rate of organic matter. The economic efficiency and subsidiary energy input are higher in "high bed and low ditch" farmland ecosystem than in paddy field ecosystem, but the conversion efficiency of subsidiary energy is far lower. The output/input ratio of total energy is rice 10.13>sugar 3.43> banana (Musa sapientum) 2.34>banana(M.nana) 1.63>citrus 0.92. The utilization rate of light energy is sugarcane 1.4606%> rice 0.9326%>citrus 0.7491%> banana (Musa sapientum) 0.5270;b >banana (M.nana) 0.4433%.

Key words: Constructed wetland, Ecological engineering, Industrial wastewater