Abstract: Stability is an important characteristic of anaerobic wastewater treatment systems. The definition and measuring parameters of stability are discussed. Moreover，the mechanism and behavior of the bioreactor operational instability caused by load shock，as well as the influence factors of stability are analyzed. The results show that the load shocks are supposed to greatly influence the operation of anaerobic reactors. The typical responses of anaerobic reactors to load shock include a decrease in performance，accumulation of volatile fatty acids，drop in pH and alkalinity，change in biogas production and composition，and sludge washout. The tolerance of anaerobic reactors to shocks is affected by the factors related to the treatment system as well as the variation of load. Accordingly，some proposals on the enhancement of stability are put forward.