Abstract: Condensation Repression by Isotope Selective Laser Activation (CRISLA) method is the most potential LIS method for industrialized application. During the separation procedure of S isotope by CRISLA method, a kind of low temperature nozzle device is introduced to obtain low temperature supersaturated SF₆ gas. On one hand, the low temperature can reduce the spectral overlap of S isotope; on the other hand, the laser represses the condensation of target SF₆ molecular and the carrier gas Xe molecular, which can gain the dimer of nontarget SF₆ and Xe, this phenomenon is beneficial for isotope separation. The calculation and optimization of actual performance of this kind of method is determined by not only the parameters of laser, but also the flow condition inside the low temperature nozzle device, to study the effect of structure of low temperature nozzle device on separation performance, which is the instruction of optimization of separation performance, it is necessary to study the flow field inside the low temperature nozzle device. CFD method is used to simulate the flow field inside the low temperature nozzle device. The distribution of flow parameters especially the distribution of temperature both in and out the nozzle is obtained. Through the results of the simulation of flow field, the designing scheme of nozzle is verified and a preliminary interpretation to the fundamental principles of isotope separation by low temperature nozzle device is proposed.