The embodiment of the invention provides an air quality predicting method and an air quality predicting device. The method comprises the following steps: first, acquiring the air quality change law within a preset time range before a target date; then, acquiring the air quality historical data which are associated to the target date according to the air quality change law within the preset time range before the target date; next, calculating the prior probability that each air quality condition appears on the target date and the corresponding predicting reliability; later on, calculating the posterior probability that each air quality condition appears and is predicted accurately according to the prior probability of the corresponding air quality condition and the corresponding predictingreliability; finally, generating an air quality predicting result according to the calculated posterior probability, and transmitting the air quality predicting result to a user terminal. According tothe scheme, the air quality within a period of time in the future can be predicted without arranging an air quality monitoring point, and a new direction is provided for predicting and forecasting air quality numerical values.