Abstract: Liquid particle measurement technology is based on light scattering method or photo resistance method to analyze and detect the particle size and quantity of the particles contained in the liquid. Aiming at the requirements of the measurement accuracy of liquid particle measurement technology, based on the principle of hydrodynamic focusing, the sheath flow system model for the measurement of particles in the liquid is designed by using SolidWorks software. Through the fluid calculation module of COMSOL, the fluid simulation calculation of the model was carried out, and the change of the focusing flow width and the focusing performance of the system on the particles were simulated. The simulation results showed that the focusing performance of the sheath flow system on the sample flow was consistent with the theoretical derivation, and the sample flow speed was 0.1m /s, and the sample flow width could be focused to 0.24mm. COMSOL particle tracking module is used to simulate the particle trajectory at this width, and the effective focusing of particles is realized. On this basis, the state of particles flowing through the detection area is analyzed, and the influencing factors of coincidence error are obtained, which has certain theoretical significance for the practical design of the sheath flow system of liquid particle measurement technology.