The invention provides an automatic control method of an air purifier. The air purifier comprises a pollutant concentration detecting device, a purifying device and an air blowing device. The automatic control method is characterized by comprising the following steps: detecting the concentration Qs of pollutants in air; detecting the concentration Qe of the pollutants in the air after a purification time T; detecting the mass Q of the pollutants adsorbed by the purifying device within the purification time T; calculating equivalent indoor volume V, wherein a calculating method is as follow: V=P/(Qs-Qe). According to the automatic control method provided by the invention, through calculating the equivalent indoor volume V, an actual rotating speed n can be adjusted according to the equivalent indoor volume V, and the purification efficiency of the air purifier is enabled to be basically the same under the situations of different purification space and adjustment of the rotating speed and the air purifier, so that the purification effect and the balance in aspects of energy saving, noise reduction and the like can be obtained; in addition, correction on the rotating speed by real-time purification efficiency of the purifying device is also considered, so that the rotating speed matching is further optimized.