0.90 m/s。3-D topography of white light interference instrument was used to observe the surface topography of biofilm under different velocity rates in PE pipe, and the impact of velocity rate and surface topography of biofilm was analyzed. The results showed that more solid particulate was on biofilm surface with roughness in the middle-velocity zone. In the high-velocity zone, biofilm surface was smoother and biofilm’s thickness was thinner. The relationship between the average thicknesses of biofilm with velocity rate agreed to the exponential law whose maxima velocity rate is 0.22m/s. Under the condition of velocity rate is greater than 0.90m/s, the growth rate of biofilm was decreased faster with the increase of velocity rate, and more biofilm fell off, and hydraulic shear stress force increased rapidly, so cleaning velocity rate of the drip irrigation system should be larger than 0.90m/s.