The bad dust collector includes circular casing, patterned plate inside the casing, cloth bag on the patterned plate, track and rotary unit. The rotary unit includes two channels comprising two rotaryarms and two oscillating arms connected separately to the rotary arms, the lower openings of these two oscillating arms operate via track wheels on the track to align to the upper mouths of the clothbags, and the upper mouth of one channel extends beyond the upper part of the casing for connection with the reverse blower towards the cloth bag to clear ash while the other channel extends beyond the lower part of the casing for connection to the exhauster. Whether some cloth bag is damaged may be judged based on the exhausted gas sample for direct replacement.