NOVELTY - A device for producing marsh gas by fermenting mixed straws comprises a fecal sewage pre-treatment tank (1), a household garbage pre-treatment tank (2), and a straw pre-treatment tank (3); where the straw pre-treatment tank is a closed case with heater installed on its bottom, micro-pored rotary case (4), and biogas spray header (5) arranged inside; the three pre-treatment tanks are connected with a main anaerobic fermentation basin (13); and an agitator (12), a circulatory heating pipe coil (14), and a temperature probe (15) are installed inside the main anaerobic fermentation basin (13).    USE - The device is useful for producing marsh gas by fermenting mixed straws (claimed).    ADVANTAGE - The device can simultaneously process the plant straws, household garbage from the kitchen, and fecal sewages; thus, achieving automatically thermostatic control and secondary fermentation with high efficiency of producing marsh gas.    DETAILED DESCRIPTION - A device for producing marsh gas by fermenting mixed straws comprises a fecal sewage pre-treatment tank (1), a household garbage pre-treatment tank (2), and a straw pre-treatment tank (3); where the straw pre-treatment tank is a closed case with a heater installed on its bottom, a micro-pored rotary case (4), and a biogas spray header (5) arranged inside; the three pre-treatment tanks are connected with a main anaerobic fermentation basin (13) via a transport tube with a sludge pump; an agitator (12), a circulatory heating pipe coil (14), and a temperature probe (15) are installed inside the main anaerobic fermentation basin (13); the main anaerobic fermentation basin is connected with a secondary anaerobic fermentation basin (16) through the transport tube with the sludge pump; the volume ratio of the main anaerobic fermentation basin to the secondary anaerobic fermentation basin is 1:1.5; the secondary anaerobic fermentation basin is connected with a sedimentation tank (7) through the transport tube (9) with the sludge pump; the bottom of the sedimentation tank is connected with the main anaerobic fermentation basin through the sludge pump installed transport tube, and the upper part is connected with the biogas spray header in the straw pre-treatment tank through the sludge pump installed transport tube (6); the other side on the upper part of the sedimentation tank is connected with an oxidizing pond (8) for processing before discharging; and the upper parts of the main anaerobic fermentation basin and the secondary anaerobic fermentation basin are connected with a marsh gas purifier (10) and a marsh gas storage device (11), connecting with user terminal.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the device for producing marsh gas by fermenting mixed straws.    Fecal sewage pre-treatment tank (1)    Household garbage pre-treatment tank (2)    Straw pre-treatment tank (3)    Micro-pored rotary case (4)    Biogas spray header (5)    Sludge pump installed transport tube (6)    Sedimentation tank (7)    Oxidizing pond (8)    Transport tube (9)    Marsh gas purifier (10)    Marsh gas storage device (11)    Agitator (12)    Main anaerobic fermentation basin (13)    Circulatory heating pipe coil (14)    Temperature probe (15)    Secondary anaerobic fermentation basin (16)