Water pollution of cities is not only an important factor that limits the development of social economy but also an important social problem which would threaten the healthy of people. Under the circumstance that point source pollution is basically controlled, non point source pollution has already become an important pollutant source. The paper here analysed the danger of non point source pollution after the discussing on the courses of non point pollution of water source and presented some prevention and control countermeasures of non point source pollution.