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 requirements of the measurement accuracy of liquid particle measurement technology, based on the hydrodynamic focusing principle, the sheath flow system model for the measurement of particles in the liquid was designed by 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 show that the focusing performance of the sheath flow system on the sample flow is consistent with the theoretical derivation, and it can be focused to 0.24 mm width when the sample flow speed is 0.1 m/s. Meanwhile, the COMSOL particle tracking module was used to simulate the particle trajectory at this width, and the effective focusing of particles was realized. On this basis, the state of particles flowing through the detection area was analyzed, and the influencing factors of coincidence errors were obtained, which had certain theoretical guiding significance for the practical design of the sheath flow system of liquid particle measurement technology.