Bag type air filter

The utility model relates to a bag type air filter which is applied to air purification places utilizing centralized air ventilation systems and is used for cleaning dust. The bag type air filter is mainly formed by an outer frame, a plurality of inner frames arranged in the outer frame, filter bags adhered and fixed onto each inner frame and a limit pulling strip. The bag type air filter is characterized in that pulling strings are arranged in the filter bags, the length of the pulling strings is sequentially reduced from bag openings to bag bottoms, and the pulling strings are arranged in spring type folded grads mode, so that ventilation surfaces of the filter bags can be trapezoid-shaped. The bag type air filter adopts a pulling string structure. When the air filter is used, resistance of the filter is greatly reduced, running energy consumption is saved and service life of the filter is prolonged.