Abstract: The density of LiF-CaF₂/YbF₃-Yb₂O₃ molten salt system is an important parameter for preparing rare earth Yb alloy by electrolysis. In this paper, Archimedes method was used to measure and analyze the density of LiF-CaF₂/YbF₃-Yb₂O₃ system and its changing rule in the temperature range of 1 173 - 1 573 K. Meanwhile, mathematical models were used to estimate the density of Yb-Ni alloy and analyze its separation characteristic between molten salt and alloy during electrolysis. The results showed that the density of the system (LiF-CaF₂)_eutand (LiF-YbF₃)_eut system decreased linearly with higher temperature, and the molar volume increased linearly with rising temperature. Moreover, the excess volume of (LiF-YbF₃)_eut system decreased linearly with the temperature. In the range of Yb₂O₃ ratio from 0 to 4wt% with the temperature ranging between 1 173 and 1 573 K, the density of (LiF-CaF₂)_eut-Yb₂O₃ and (LiF-YbF₃)_eut-Yb₂O₃ molten salt systems decreased linearly as the temperature rised. When the temperature was constant, (LiF-CaF₂)_eut-Yb₂O₃ system increased linearly in the range of 0≤x(Yb₂O₃)≤2.5%, while (LiF-YbF₃)_eut-Yb₂O₃ system increased linearly in the range of 0≤x(Yb₂O₃)≤3.5%. The density of Yb-Ni alloy was higher than that of LiF-CaF₂/YbF₃-Yb₂O₃ molten salt system so that it was easier to be separated by electrolysis.