Abstract: Taking the design of Beijing Wenyu River biological corridor as an example,the authors put forward that the key of building an ecological city is to improve the city’s control ability of environmental quality.There are two aspects that should be highlighted in the design.On one side,the natural component with the plants as the main body,with proper heterogeneity and with some amount of area should be regarded as controlling component and be kept existing persistently.On the other side,the "species flow" should be guaranteed to move smoothly in the city landscape.