To promote the development of condensation particle counter (CPC) and improve its detection performance, the research of flow field simulation and flow ratio optimization in CPC particle grower are carried out systematically. The basic structure and analytical model of the particle grower are designed and established. The parameter distribution in the flow field of the particle grower is investigated by Fluent software simulation. Meanwhile, the effect of the flow ratio on the variation of saturation and the theoretical counting efficiency of CPC is considered. Simulation results show that the sheath structure can restrict the aerosol particles to flow at the position with high saturation in the central axis of the flow field. The higher the flow ratio is, the smaller the diffusion range of aerosol particles is. In a reasonable range, the increasement of the flow ratio can increase the vapor mass fraction and saturation, and the critical diameter is reduced. Under the setting of optimized flow parameters, the particle diffusion range of self-designed particle grower is only 4 mm, and the maximum value of saturation can exceed 7. The self-designed CPC measurement system is designed according to the numerical simulation information, and the calibration experiment of its counting efficiency is carried out. The calibration results show that the relative errors of 13 points within the particle size range of 7~200 nm are all lower than 9% , which fit the calibration specifications well. Thus, the design of CPC structure and optimization of operational parameters based on the numerical simulation is instructive for CPC development.