Abstract: In order to study the probability distribution of counting signal amplitude in a laser airborne particle counter,the relationship between the probability distribution of counting signal amplitude,particles number density in the sampling air-flow, light-intensity uniformity of the sensitive volume and the speed distribution of the lamellar flow were theoretically analyzed. Then, the probability density distribution of the counting signal amplitudes was tested with a self-designed optical sensor with sheath-air inlet, and it matched well with the theoretical result. In a word, the results indicate that the probability distribution of counting signal amplitude is determined by both the intensity distribution of the sensitive volume and the space distribution of the particles number density, which proposes a theoretical basis for the design of the particle counter.