The invention discloses a biological filtration device, which comprises a front pool and a biological filtration bed, the front pool comprises an adjustment pool for preliminary sewage treatment, a refluxing pool for receiving refluxed water from the biological filtration bed, a water-outlet sand-filtering pool for filtering the of the refluxed water from the refluxing pool for a second time; an ejector arranged on the ground floor for mixing the refluxed water from the refluxing pool and the sewage from the adjustment pool, and discharging the mixed water into the biological filtration bed; the biological filtration bed comprises a shell, a filling device which is arranged in the shell and is used for treating the sewage, an inlet connected with the outlet of the injector, an outlet connected with the refluxing pool; the biological filtration bed provides an air inlet and an air outlet for ventilation. The device is arranged underground, but the wearing parts are installed on the ground, therefore, the maintenance is convenient, and the device has strong capability to treat wastewater and does not cause sludge blocking.