Wastewater irrigation test under biological swimming bed and biological filter was done to study biofilm growth situation in emitters. Results showed that biofilm growth experienced three periods of attachment period, growth period and maturity period, and biofilm was at a dynamic equilibrium state in maturity period. Biofilm was attached fast, growth fast and was the thickest on the inlet of emitters. Detached biofilm deposited on the inside flow or the outlet, which was embodied on the flow changes. The average flow changes of emitter lagged growth change of biofilom. When biofilm reached maturity period, it was detached repeatedly to grow, and this was high-risk period of emitter clogging when the average flow of emitter was declining. After 184h, acid and chlorine can be added to flush emitter.