The hydraulic residence time (HRT) is a key parameter affecting the water treatment process, concerning water quality guarantee, short HRT will shrink capacity of structure, in order to reduce capital construction fee and operation cost. Filtration experiments using a bench-scale immersed membrane separation system combined with a high dose of PAC (powdered activated carbon) were conducted in order to investigate the effect for organic matters, pesticides and ammonia-nitrogen, and choose the optimal hydraulic retention time. The experiments were carried out under such operation conditions as a filtration rate of 0.5m/d, water temperature of 20°C. Synthetic water was used as the source water; the average influent concentrations of TOC (total organic carbon), and NH3-N (ammonia nitrogen) were 7.8mg/l, 6.8mg/l, respectively，201μg/L for THMFP and 201μg/L for Nitrobenzene. The results illustrated that, when HRT reduced from 3h to 1h, the removal rate of TOC, UV254 decreased a little, so did ammonia-nitrogen. But it had little effect on THMFP and nitrobenzene. When HRT was 1 hour, the average removal rate of TOC was 75%, 92% for ammonia-nitrogen, 97.5% for nitrobenzene and 75% for THMFP, which was all less than that of GB5749-2006 and that’s the optimal hydraulic retention time.