A portable air purifier disclosed is advantageous in that the structure of the air purifier allows primary pollutants to be filtered by a pre-filter at the same time of suction of outside air by an air compressor, and allows the filtered compressed air to be ejected into water through a siphon and a diffuser, and therefore ultrafine dust in the air is subjected to a second time of separation, finally pure humidified air purified is supplied through a casing pipe to a user's nasal cavity or oral cavity, the maintenance cost for changing the special filter regularly is omitted, and inner configuration elements are reduced so that the size is reduced, thus reducing the production cost and making the air purifier portable and convenient.