The density of molten salts is the most important property in the development of molten salt reactor (MSR). The density value measured through the experiment is also very valuable as a gold standard for the validation of the prediction models based on molecular dynamics or other computational methods. To the best of our knowledge, the experimental density data of the ternary NaCl-MgCl2- UCl3 salt system as a MSR candidate fuel salt have never been reported previously. In this study, density measurement experiment of high-temperature molten salt of NaCl-MgCl2 and NaCl-MgCl2- UCl3 was conducted using a previously-developed density measurement system based on the maximum bubble pressure (MPB) method. As a result of the experiment, the density value of 62NaCl- 18MgCl2-20UCl3 molten salt at 873 K was 2.62 g/cm3. A density prediction value of 2.65 g/cm3 at 873 K was derived from the obtained results based on the rule of additivity of molar volume method. The predictred density of 62NaCl-18MgCl2-20UCl3 was consistent with the experimental value within 1%. The density measuring system used in this study is promising for the validation of other multicomponent molten salt systems.