The invention discloses a low-efficiency air dust filter. The low-efficiency air dust filter is provided with a fixed blade with a swirling flow guiding function and a fin blade capable of rotary separation of dust and impurities at a high speed. According to the adopted scheme, the low-efficiency air dust filter comprises a semi-closed cylinder casing (1), the fin blade structure (5) capable of rotary separation of the dust and the impurities at the high speed, an inflow air swirling flow guiding device (8) and a double-bearing structure shaft (11). When outside air with the dust and the impurities enters the casing (1) from the bottom air inlet of the inflow air swirling flow guiding device (8) through swirling flow guiding blades (9), the formed swirling flow drives the inside fin blade structure (5) to conduct high-speed rotation, the dust and the impurities are discharged from a dust outlet (3) of the casing (1) under the function of poking pieces (7), and the pre-filtered air passes through a coarse filtration air outlet (10) and enters lower-stage filtration through a lower-stage filter connector (12). The low-efficiency air dust filter has the characteristics of being simple in structure, high in dust-removing efficiency and simple to maintain.