The effect of hydraulic retention time (HRT) on the performance of a combined packed sequencing batch biofilm reactor (SBBR) for treating simulated domestic wastewater was investigated.The changes of denitrification performance,denitrification rate and EPS in SBBR were detected and analyzed at different HRT (16 h, 12 h and 8 h).The results indicate that HRT has no significant effect on COD removal rate.Shortening HRT inhibited the activity of microorganisms on the biofilm,increased the mass fraction of TB?EPS and LB?EPS by 30.25% and 29.88%,and decreased the denitrification rate to 64.1%.The best HRT obtained from the experiment was 12 h.