Abstract: Abstract: It was reported that in China the annual consumption of edible oil was 21×106 tons which consequently produced waste oil of about 6×106 tons. The waste oil can be recycled to produce biodiesel, bulk chemicals, surfactant, fine chemicals and biogas. The recycling and bioconversion of waste edible oil into biogas will be beneficial to energy crisis and environmental pollution, and promote sustainable economic development. Because of the suppression of long chain fatty acid that is one of intermediate metabolites, waste edible oil that is directly used in anaerobic fermentation often causes digestion inhibition or system crash. The two-phase anaerobic digestion process can avoid the accumulation of long chain fatty acid and volatile acid in the methanogenesis process. Waste edible oil hydrolysis and long chain fatty acid degradation are critical steps in anaerobic digestion of waste edible oil. Improving the production rate of volatile fatty acid (VFA) in hydrolysis acidification phase will raise the subsequent methanogenesis reaction. The response surface methodology (RSM) can obtain the best combination of the factors in a given area and the optimal value of response values, which has been widely used in the optimization of culture conditions and technology conditions in the areas of food processing, water treatment and so on. Therefore, the RSM was used to optimize waste oil biological hydrolysis to produce volatile acid (including acetic acid, propionic acid and butyric acid). By adopting the method of central composite design (CCD) line design of experiment, using the soft Design Expert to analyze the experiment result, we established the quadratic polynomial in which volatile acid concentration was taken as the response value. The effects of initial pH value, raw material load, reaction time and inoculation rate on volatile acid concentration were investigated. The mathematical model and optimized parameters of process were also acquired. The results showed that the effects of different factors on volatile acid concentration were as following: inoculation rate>reaction time>raw material load>initial pH value. The mathematical model could greatly simulate the practical process with the equation F value of 15.65, correlation coefficient of 0.9359 and adjusted correlation coefficient of 0.8761. The optimum parameters of biological hydrolysis process of waste edible oil were initial pH value of 6.2, raw material load of 300 g/L, reaction time of 8 d and inoculation rate of 40%. Under the optimum conditions, the volatile acid concentration was up to 7221.0 mg/L which was closed to the predicted value of 7224.0 mg/L. The anaerobic fermentation was carried out using waste edible oil with or without acidification treatment. Under the optimum condition, the methane content and total methane yield after acidification increased by 44% and 11%, respectively, and also, the removal rates of chemical oxygen demand (COD) and volatile solid (VS) increased by 28% and 51%, respectively. Furthermore, the time for achieving 80% of total gas production during a cycle of anaerobic fermentation was shortened by 3 days.