Abstract: The hydraulic characteristic of the anaerobic baffled reactor (ABR) is an important factor affecting its treatment effects. Hydraulic retention time (HRT) has a greater influence on the hydraulic characteristic of the reactor. A double flow model and a k-ε turbulence model have been used to simulate the three-dimensional flow field in the anaerobic baffled reactor at different HRT and analyze the influences given by HRT on the hydraulic characteristic of inner flow field of the reactor from both water flow velocity and solid holdup. The analysis results show that HRT has a little influence on the flow velocity of the down-flowing compartment, however, there is an increase in the velocity of circulating water flow with decrease in HRT in the up-flowing compartment. HRT has a great influence on the distribution of the solid holdup. Either the shorter HRT or the longer HRT has a negative effect on the improvement of the efficiency of the reactor. The optimal hydraulic retention time of the reactor is 5~8 h, which is the most favorable for the removal of pollutants in the waste water.