To study the effect to water purification ability, wetland plants and algae groups of surface-flow reed wetland by adding biofilm,six different density of biofilm carriers were selected to construct six surface-flow reed wetlands. Their removal rates of major pollutants and the change of DO and turbidity were compared ,the effects to reed,periphytic algae and floating algae of wetlands were also investigated. The data generated by this experiment was analysed by a software called SPSS. The results showed that adding biofilm significantly improved the purification capacity of surface-flow wetland. The removal rates of CODCr,TN,TP and ammonia nitrogen were increased by 4.7%~12.8%,3.2%~11.9%,-1.3%~7.6%,5.9%~11.7%,respectively. The removal rates of pollutants were positively correlated to the quantity of biofilm. Also,DO and turbidity of water were obviously affected by biofilm. Significant improvement of DO and turbidity was seen when the density of carriers was high(≥3.3 per square meter) and the turbidity had an obviously linear negative correlation with the quantity of biofilm. In addition,adding biofilm had no significant effect on the growth of reed,density and structure of periphytic algae but may promote the photosynthesis of floating algae.