Abstract: Protecting water or consuming water is an unsure issue on virescence plant. To study the influence of plants in the cycle of water，especially for Beijing ,which is lack of water seriously and expand the green plant to beautify the city and get ready for the Olympic Games in 2008. By inquiring data about the water resource, precipitation and transpiration of Beijing in recent years, while analyzing them, found that there were some improvement for the humidity of the air and the water source was protected in some extent. But the overall water source is decreasing, which should be attached importance to.