Abstract: Two-phase anaerobic biological treatment has been widely applied in waste disposal/utilization. In order to study the effects of main products on the biogas-producing process, based on the simulation of four major acidified organic acids (formic acid, acetic acid, propionic acid and butyric acid), the activated sludge was inoculated. Then single factor and orthogonal experiments were designed to study the main and interactive effects of organic acids on dynamics of both daily and accumulative methane production under medium temperature condition. The results showed that there existed a critical threshold of organic acid concentration in single organic acid, where high concentration above the threshold would have inhibited effect on gas production; and gas production got increased with the acid concentration increasing when it below the threshold. There was a significant interactive effect among the four organic acids, and acetic acid had the stronger increasing effect than butyric acid, propionic acid had the constrained effect. Acetic acid had best effect on gas production among the four organic acids under same concentration. Formic acid got the highest gas yield and had less inhibiting effect than propionic acid. It is suggested that the fermented material concentration, which easily produce organic acids, should be adjusted in order to improve gas production efficiency.