Electrical Low Pressure Impactor is a measurement device, which can real-time measure the distribution of particle size and concentration. The PM10 cyclone separator is used as the pretreatment device of the Electrical Low Pressure Impactor, so the separation performance of the cyclone separator is the key factor to measure the particle concentration accurately. ICEM was adopted to mesh, and gas-solid two-phase flow of the separator was studied based on Reynolds stress model and the stochastic model. The result shows that the internal flow field in the preseparator is the force vortex state, the outer swirl is the composite vortex structure of quasi free vortex, the composite vortex has a good effect on particle separation. The change of dust discharge diameter adjusts the critical diameter to the theoretical value, which making more particles whose size larger than 10μm were collected to the maximum extent, which provides a guarantee for the accurate measurement of particle concentration.