Abstract: During the countercurrent anaerobic fermentation of cassava dregs, the effects of different substrate concentrations and substrate residence time on of acid production and degradation rate of the substrate were investigated. The best substrate concentration is 80 g/L, and the best sludge retention time is 12 d. Under this condition, the concentration of VOA reach 6.75 g/L,and the ratio of acetic acid to VOA reach 75.9%,and substrate degradation rate reach 27.1%.