The invention discloses a back-flushing filter unit used for a plastic extruder and a back-flushing method of the back-flushing filter unit. The back-flushing filter unit comprises a filter unit body (1) connected with the discharge end of the plastic extruder. One side of the filter unit body (1) is provided with a feed opening (101), and the other side of the filter unit body (1) is provided with a discharge opening (107). A filter assembly used for filtering a material flowing in the filter unit body (1) through the feed opening (101) is arranged in the position, located between the feed opening (101) and the discharge opening (107), of the filter unit body (1). The filter unit body (1) is further provided with a back-flushing assembly used for discharging impurities filtered out by the filter assembly. The back-flushing filter unit has the advantage that impurities are discharged thoroughly.