Of all the challenges facing the sustainability of cities, water, food and energy are the most critical. In the context of rapid urbanization, unsustainable human activities have resulted in fundamental changes in the structure and function of land cover and urban water systems, and the degradation of ecosystem services. Taking Beijing - a typical fast-growing mega city - as an example, to establish a food-energy-water impact model, this research studied the temporal-spatial changes in the water system pattern in a mega city, along with the driving forces, especially the nexus to rice production and energy, and the ensuing series of environmental impacts.On the basis of land use data, remote sensing images, and thematic maps from 1993, 2001 and 2007, water system information was extracted and adjusted for Beijing. With the aid of RS and GIS techniques, the water system was classified into four types, and the spatial and temporal dynamic of the landscape patterns of Beijing’s water system systematically was analyzed. The landscape metrics were then calculated using FRAGSTATS 3.3. The results show that the total area of water system in Beijing declined from 63,494 ha to 43,652 ha from 1993 to 2007. The decrease of the linear water surface is more significant than that of the non-linear water surface. In the terms of landscape metrics change, the number of patches has decreased from 5510 to 5396. The density indexes have increased by 40.61%, the average area of patches has decreased by 30.18%, the patch shape has tended to become more regular, and the overall pattern of the water system is becoming more fragmented. Urban sprawl, the shortage of water resources, and the increasing amount of construction land are the major reasons accounting for the changes in the water system in Beijing. Because of these changes, farmland has decreased by 212,428 ha, and especially rice paddy fields, are significantly decreasing from 52,200 ha in 1980 to 199.6 ha in 2015, a decrease of 99.62%. Consequently, rice production is decreasing by 99.61%, more energy is being consumed for food production. Meanwhile, the local water supply rate has decreased from 100% to 78.4% between 2001 to 2016, the underground water level has decreased by 14.24 m from 1994 to 2016. There is more competition for water resources, more urban water flooding disasters, and emerging urban environmental problems such as declining underground water level, posing a serious threat to the sustainability of the city. Therefore, a systematic and smart thinking is needed to analyze the complex land use - water- food – energy relationship.