An air filter has a housing, a filter core, a resilient abutting element, a sleeve, a threaded rod bracket, a threaded rod and a nut. The axial sealing is achieved by axially compressing the resilient abutting element, such that the user does not have to apply much physical effort to compress the resilient abutting element. The user can easily push the filter core into the housing, and then rotates and tightens the nut, which can axially compress the resilient abutting element. It is easy to apply force on rotating the nut, and better still, the nut can be rotated by an electric tool, which further facilitates the convenience in assembling. Besides, the nut only abuts the sleeve, and the threaded rod bracket does not abut the filter core. Consequently, the filter core is not blocked, thereby making the air smoothly pass through the filter core.