In order to study the degradation characteristics of the deinking wastewater by the down flow anaerobic sludge bed (DASB) reactor, the deinking wastewater which was from a paper mill was subjected to anaerobic treatment using a downflow anaerobic sludge blanket reactor. During the period, the changes of chemical oxygen demand (COD) mass concentration, COD removal rate, pH value, mixed liquid suspended solids (MLSS) value and the characteristics of anaerobic sludge were investigated to reveal the degradation characteristics of the reactor. The results showed that the removal rate of COD in the DASB reactor during the start-up phase was stable at about 62%, and the start-up effect was better. The average removal rate of COD was higher in the load operation stage. When NaHCO₃ was introduced in the influent water, the acidification phenomenon of the reactor can be adjusted. During the operation, the MLSS of each compartment slowly increased, and finally reached a stable state. Through this test, the startup process of the DASB reactor under ambient temperature conditions is clarified, and the operation is better controlled to optimize the process. In addition, this study provides an important basis for the treatment of organic wastewater in DASB reactor.