Abstract: In order to reduce the impact induced by the difference in refractive index between the working fluid and fuel assembly model on the visualization flow field measurement, tetralin and ethyl alcohol were mixed to produce a new refractive index matching (RIM) liquid. The refractive index, density, dynamic viscosity, and Reynolds number of the mixed solutions with different concentrations were measured, and the sensitivities of these four variables to temperature were investigated. The results show that, the refractive index of the mixed solution of tetralin (72.2%)-ethyl alcohol (27.8%) is equal to that of polymethyl methacrylate at 25 ℃, and the Newton equation has the smallest deviation from experimental data when the refractive index of mixed solution is predicted. Since the Reynolds number is sensitive to temperature, it is suggested to set up heat exchange equipment in the experimental facility to control the temperature of the working fluid. This RIM liquid has the advantage of low cost, moderate density, low dynamic viscosity, and high stability, which can be used in the flow field visualization experiment.