Laser airborne particle counting sensor (LAPCS), based on light scattering of particle, is specially used in clean environment monitoring. LAPCS samples the air by a pump, and uses a laser illuminating the sampled air in the chamber, then counts the total number of scattering signal and its amplitude distribution, which can characterize the number of particles and size distribution. The structure of air-flow-path in LAPCS directly influences the flow of sampling air, the particle trajectories and velocity distribution in chamber that will influence the performance of LAPCS. In this paper, a finite element arithmetic based on Ansys Fluent14 software environment was developed to simulate the air flow and particle flow in LAPCS. Based on numerical calculations, velocity distribution of airflow and particle trajectories in chamber of LAPCS with different nozzles are presented intuitively. A few particles probably are disturbed outside the air-flow path and pass the photosensitive area many times, which can make the LAPCS iteration count. The results can provide a theoretical basis for optimizing design of the LAPCS.