Abstract: The effects of the bleaching powder and trichlorphon on the number and activity of fungus in the biofilm attached on the solid surface were studied. The analysis of BOD5 curve was performed, and the numbers of bacterial, actinomycetes and fungus, CODMn, ammonia N concentration and the observations of electron microscope before and after the experiment were measured. The experiment results show that the activity of biofilm could be restored when bleaching powder mass concentration is below 6.17 mg/L. The activity could be restored up to 80% when the bleaching powder mass concentration is 0.7 mg/L for 15 d. With the experimental concentration, the influences of trichlorphon on the activity of biofilm and water purification ability were not obvious, but it should be used carefully to avoid its polluting on the environment.