Nonylphenol (NP) biodegradation under anaerobic conditions is difficult. Here, enhancement of anaerobic NP biodegradation mainly by regulating the role of acidogenic bacteria during anaerobic fermentation of waste activated sludge (WAS) for short-chain fatty acids production is reported. The maximum degradation efficiency of NP (69.4%) was achieved under conditions of pH 10.0 and 10 mg/L Brij 35 within 8 d, which was nearly 3-fold of that in the control (24.6%). Mechanism exploration revealed that the bioavailability of NP and specific NP-degrading bacteria and their functional genes were advantageous to NP biodegradation with alkaline pH and surfactant. More importantly, acidogenic bacteria, the dominant functional bacteria in WAS fermentation systems, were demonstrated to be involved in NP anaerobic biodegradation by providing intermediate organic substrates, as well as through their intrinsic NP-degrading abilities. Possible pathways of NP biodegradation assisted by acidogenic bacteria during anaerobic fermentation were also proposed based on the detected metabolites.