A particle detecting device is provided. The particle detecting device includes an impactor, a resonator and a piezoelectric actuator. A gas containing a plurality of suspended particles is transported into the impactor and is impacted by the impactor for performing separation and screening based on different diameters of the suspended-particles. Moreover, screened and required-diameter particles from the impactor are collected by the resonator to detect a mass and a concentration of the screened and required-diameter particles. Thus, the air quality can be monitored anytime and anywhere.