An autoradiographical method using an imaging plate (IP) was developed to identify rapidly the density of plutonium (Pu) particles on filter samples. Photostimulated luminescence (PSL) signals were obtained by exposing IPs to filter samples collecting Pu particles and naturally occurring radon decay products, respectively. Search and analysis of the alpha-induced PSL signals presented the time-dependent growth of PSL intensity from the Pu particles and the range of PSL intensities from radon decay products, making it possible to discriminate Pu particles from radon decay products using an empirical Pu-discrimination level, and then to quantify the Pu activities on a particle-by-particle basis. The method is useful for a fast screening of filter samples for Pu particles because it requires no special skills and tedious procedures during its use, gives a low discriminable activity for individual Pu particles with a relatively short exposure time, and allows us to measure a large number of filter samples at the same time.