The water system of Yanqi Lake in Shenzhen is a typical urban internal lake. In slightly polluted state, there are the problems of excessive organic pollutants, poor sensory effect of water body, insufficient hydrodynamic force and imperfect water ecology in the water system of Yanqi Lake. In order to promote the well-being of the surrounding residents, the systematical engineering measures of controlling source, intercepting sewage, dredging sediment, containing water surface dregs, strengthening circulation and rebuilding ecology are taken to comprehensively improve the water environment of the lake. After improved, the quality of lake water environment has been steadily improved from the micro-pollution state to the Class IV water standard in the Surface Water Environmental Quality Standard (GB 3838—2002). And the mass concentration of BOD₅ and ammonia nitrogen is stable within 6 mg/L and 0.32 mg/L, respectively.