Nowadays, more and more people are concerned about air quality. The particle size of aerosol is an important parameter of air quality. The classical laser particle size analyzer has some disadvantages, such as high price, large volume, and difficult processing. We introduce the scattering receiving screen into the forward light intensity distribution detection system, which only needs a commonly available CCD to obtain an image. Our method has the advantages of low cost and large detection angle. A genetic algorithm was then used to retrieve aerosol particle size distribution from scattered light intensity images. The experimental results show that the aerosol particle size distribution measured by the imaging method is close to that provided by a commercial particle size spectrometer. Moreover, the experimental results also show that this method can measure a polydisperse system. The method presented in this paper provides a simple and feasible scheme for the online detection of particle size distribution in atmospheric, medical, and industrial environments.