The instrument is a multi-stage and critical orifice cascade impactor, used to measure particle size distribution and total content degree of various solid and liquid particles in the air. At a certain airflow velocity, some particles with higher inertia will impact collection plate, while particles with smaller inertia will go with airflow and enter into the next stage. Particles with different aerodynamic equivalent diameters will impact on collection plate under certain conditions of orifice geometry and airflow velocity.