NOVELTY - Producing biogas and fatty acids, comprises adding anaerobic microorganism and culture medium into a fermentation system, controlling the fermentation system, adding organic substrate into fermentation system, increasing the temperature of the reaction system, adding organic substrate in a sequencing batch mode or a continuous flow mode into fermentation system, where the organic substrate is mixed with water in a concentration of 0.5-30 g/l, where the system has a hydraulic retention time of 2-30 days.    USE - The method is useful for producing biogas and fatty acids (claimed).    DETAILED DESCRIPTION - Producing biogas and fatty acids, comprises: (a) adding anaerobic microorganism and culture medium into a fermentation system, and controlling a rotating speed of the fermentation system to 300-450 revolutions per minute, a pH value to be 6.5-7.5 and an initial temperature to 20-55 degrees C to perform primary fermentation, (b) adding organic substrate into the fermentation system according to the program, maintaining the concentration of the organic substrate to 0.05-0.2 g/l, gradually increasing the temperature of the reaction system until the fermentation temperature of the system reaches to critical temperature, and performing the process for 20-40 days; (c) keeping the system at constant temperature, and continuously adding organic substrate in a sequencing batch mode into the fermentation system for maintaining the concentration of the organic substrate at 0.05-5 g/l, and performing the process for 8-15 days; and (d) adding the organic substrate in a continuous flow mode into the fermentation system, where the organic substrate is mixed with water in a concentration of 0.5-30 g/l, where the system has a hydraulic retention time of 2-30 days. An INDEPENDENT CLAIM is included for a device comprising a reactor, where the reactor is connected with the organic substrate supply device, a culture medium supply device, liquid phase metabolic product recovery device, metabolic product recovery device, a pH value detecting device and a pH value control device.