An air purifier includes a casing (15) with an air inlet and an air outlet. In the casing, there are disposed with a dust cleaning apparatus, a reactant generator (16) positioned downstream the dust cleaning apparatus, a valve (22) and a catalyst filtering coreing core (17) positioned downstream the reactant generator (16), and exhaust fan at any location, a dust sensor (23) and a central processing unit (24) for controlling open and close of the valve (22).