Abstract: Condensation repression by isotope selective laser activation (CRISLA) is a promising laser isotope separation (LIS) method，which has the ability to separate a wide range of isotopes. The flow field within the separation device has an impact on the separation performance. The flow in the separation device under different conditions was modelled and simulated by computational fluid dynamics software. Based on the simulation results，the effect of nozzle structure on the flow was analysed. The results showed that the throat diameter is the most important nozzle structure factor affecting the temperature of the flow, and the length of the nozzle expansion section affected the temperature of the flow more significantly in the nozzles with a throat diameter of 3 mm or less; For nozzles with a throat diameter of 35 mm, the low-temperature region of the supersonic flow is easily formed outside the nozzle，for nozzles with a throat diameter of 2 mm or less, it is necessary to optimize the length and angle of the nozzle expansion section to obtain an ideal flow field. Finally, the article provides several nozzles that can form ideal flow fields, providing some reference for future experimental researches.