Abstract: Abstract: Anaerobic digestion (AD) is a kind of biological technology, which can combine organic waste treatment and energy recovery together. AD is a complex and well-organized metabolic process with a series of ordered biochemical reactions participated by multiple anaerobic microorganisms. Compared to other microbes in AD process, Methanogenic archaea are most sensitive to environment and have the lowest metabolism rate, so the methanogenic stage is the bottleneck step of anaerobic digestion system. The characteristics of vegetable waste, such as high content of moisture and organic matter, make it suitable for anaerobic digestion. Vegetable wastes are easily perishable materials. It is difficult to realize the balance between hydrolysis/acidification and methanogenesis during the digestion of vegetable waste. The methanogens are easily inhibited by cumulative volatile fatty acids, especially under high organic loading rate (OLR). Therefore, to stably running biogas plant, the digestion system with vegetable waste as substrate is always operated under low OLR. However, the lower OLR always results in a low efficiency of digester and low treatment capacity of plant. Taking into account the efficiency and stability, it is necessary to find effective instability early warning indicators and control strategies. In order to select a set of early-warning indicators for the acidification of the digestion system, continuous anaerobic digestions of vegetable waste were carried out at OLR of 0.5, 1.0 and 1.5 g/(L·d) in the self-designed 70 L reactors under mesophilic conditions (35 ℃). During the digestion, the primary parameter of the gas and liquid phase were directly monitored by on-line analyzer (daily biogas production, CH4 and CO2 content, pH value and oxidation-reduction potential (ORP)), and off-line analytical instrument (total alkalinity (TA), bicarbonate alkalinity (BA), intermediate alkalinity (IA) and volatile fatty acids, including acetate, propionate, n-butyrate, iso-butyrate, n-valerate, and iso-valerate). Secondary parameters, such as the value of CH4/CO2, total volatile fatty acids/BA, BA/TA, IA/BA, were indirectly monitored by data processing. Four types of parameters were selected as the indicator of the instability early-warning for mesophilic anaerobic fermentation with vegetable waste as substrate. According to the early-warning ability, the four types of indicators were as followings: 1) CH4/CO2. When CH4/CO2<1.06, the system tended to be unstable. It was the best early-warning indicator for the mesophilic anaerobic digestion of the vegetable waste and could warn the instability of the system 18 d ahead of complete failure of digestion system. 2) The propionic acid, butyrate, iso-butyrate, valerate and iso-valerate concentrations. When the concentrations of these acids increased suddenly, the digestion system also tended to be unstable. The sudden increasing occurred 17, 15 and 15 d ahead for propionate, butyrate and valerate, respectively. 3) The values of BA, BA/TA or IA/BA. When BA<1 500 mg/L, BA/TA <0.8 or IA/BA>0.4 occurred 13, 12 and 12 d ahead, the buffering capacity of the system was very poor, and the system would be inhibited by accumulative volatile fatty acids. 4) ORP and pH value. The values of ORP and pH changed slowly and continuously. The absolute values of ORP and pH were difficult to be used as indicators. The relative change value, such as daily increasing of ORP or daily decreasing of pH value, could be used as early-warning indicator. The sharp increasing of ORP and decreasing of pH value occurred 5 and 4 d ahead. For engineering application, CH4/CO2 is recommended as indicator due to its convenience and on-line determination.