Abstract: An underground unpowered anaerobic reactor (UUAR) for the treatment of rural domestic sewage was studied in this paper, based on the anaerobic biomembrane technology and push-flow theory by using the anaerobic pipeline or baffler reactor full of hollow-sphere fillings. After more than one year of laboratory-scale experiment, pilot-scale test and practical application, the results showed, under the condition of hydraulic retention time (HRT) 1 d and normal temperature, the average removal ratios of COD_Cr, BOD₅, SS, T-N, T-P, coliform group bacteria, total number of bacteria and ascaris of UUAR reached 66.1%~68.3%, 70.8%~76.8%, 80.5%~90.2%, 18.26%~23.04%, 33.9%~35.2%, 95.8%~99.8%, 37.4%~82.9% and 78.7%~100%, respectively, which met the requirements of second class discharge standard of China. Furthermore, by optimized the mixture ratio of variable hollow-sphere media and extended the HRT to 2 d, the indexes of water quality of the effluent measured up to the first class discharge standard, and residual sludge was little in the reactor. UUAR has the merits such as power-off, land-free, little effect by influent variety and no operating cost, and is suitable for the decentralized treatment of rural domestic sewage.