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Aims:  The aim of the study was to investigate the feasibility of a continuous reactor for psychrophilic anaerobic wastewater treatment by using the sludge from cold natural environment. Methods and Results:  Six sludge samples (S1–S6) were collected from different cold natural locations to select sludge with high anaerobic microbial activity under low temperatures. After a 225‐day incubation, the maximum specific methane production rate of a waterfowl lake sediment (S1) at 15°C (70·5 mLCH4 gVSS−1 day−1) was much higher than all other samples. S1 was thus chosen as the seed sludge for the reactor treating synthetic brewery wastewater at 15°C, by immobilizing the micro‐organisms on polyurethane foam carriers. The chemical oxygen demand (COD) removal efficiency reached over 80% after 240‐day operation at an organic loading rate of 5·3 kg m−3 day−1, and significant enrichment of biomass was observed. Clone libraries of the microbial communities in the inoculum had high diversities for both archaea and bacteria. Along with a decrease in microbial community diversities, the dominant bacteria (79·5%) at the end of the operation represented the phylum Firmicutes, while the dominant archaeon (41·5%) showed a similarity of 98% with the psychrotolerant methanogen Methanosarcina lacustris. Conclusions:  The possibility of using anaerobic micro‐organisms from cold environments in anaerobic wastewater treatment under psychrophilic conditions is supported by these findings. Significance and Impact of the Study:  This study enriches the theory on microbial community and the application on anaerobic treatment of sludge from cold natural environments.