Abstract: In this paper, the phase diagram of molten salt system NaCl-CaCl₂-SrCl₂ has been determined by MDTA with high sensitivity, using the calculated phase diagram as a prior one. Especially, it is considered that supercooling of salts affects the accuracy of measurement.This ternary system contains an eutectic point at 41.9 mol% NaCl,40.0 mol%CaCl₂, 18.1 mol%SrCl₂, 744K. In the liquidus surface for saturation with the CaCl₂-SrCl₂ solid solution unvariant line starts at 823K, and the liquid composition is 24.8 mol%NaCl, 50.1 mol%CaCl₂,25.1 mol%SrCl₂.This phase diagram is important for the practice of sodium electrolysis. It may furnish an example in judging the reliability of a thermody-namic model that has been applied for calculation of the phase diagrams of charge-asymmetric ternary molten salt systems.