The present invention discloses an anaerobic biological wastewater treatment system for treating wastewater containing organic contaminants, which includes an anaerobic sludge bed reactor and a dissolved-air flotation tank. The anaerobic sludge bed reactor is used to decompose organic contaminants by way of an anaerobic biological treatment. The dissolved-air flotation tank is used to carry out liquid-solid separation on effluent from the anaerobic sludge bed reactor, so that anaerobic microorganisms entrained in the effluent can be recovered and recycled to the anaerobic sludge bed reactor, thereby enhancing the hydraulic loading of the anaerobic sludge bed reactor.