The utility model discloses an inner loop formula biological filter anaerobic reactor, including reaction shell, bottom and middle part in the reaction shell are provided with initial response room and second reacting chamber respectively, be provided with the solid filling bed in the second reacting chamber, the indoor water distribution system that is provided with of initial response, be provided with the gas collection baffle between initial response room and the second reacting chamber, the second reacting chamber and reaction shell between constitute gaseous rising passway, gaseous rising passway top is provided with the three -phase separator baffle, the top that is located the three -phase separator baffle in the reaction shell constitutes the three -phase separator drip chamber, reaction shell's top is provided with the air chamber, be provided with the gas vent on the air chamber, the three -phase separator drip chamber is connected with the delivery port, the initial response room is connected with row's mud mouth. The utility model discloses under the situation of external force not, the gas that utilizes initial response room self to produce can form the inner loop effect to the reaction liquid in the solid filling bed to it passes heating effect and reduces the short effect that flows to increase the mass transfer.