Lack of buffering capacity of fermentation substrate in anaerobic digestion may easily lead to acidification. Alkalinity is a key indicator for measuring buffering capacity of the fermentation substrate, so its online monitoring should be enhanced to keep the anaerobic fermentation process stable automatically. An online monitoring system based on joint titration of alkalinity and VFA was designed. The software system was introduced in detail. The system could determine alkalinity and VFA simultaneously, which achieved real-time curve display and pH meter calibration. Moreover, automatic analysis could be realized. The experiment results showed that the system had the ability of the rapid determination of alkalinity and VFA in the fermentation process. The relative standard deviation was less than 5%, which was lower than the manual titration and indicated a high measurement accuracy of the proposed system.