Background： In contrast with pressure water reactor, molten salt reactors （MSR） have low pressure, small size, high multiplication efficiency, and little residual radioactivity. The residual heat removal system is an important part of nuclear power reactor to ensure the reactor safety. However, little attention has been paid to the passive residual heat removal system （PRHRS） of the molten salt reactor in the literature so far. Purpose： This study aims to make some preliminary work for the design of the passive residual heat removal system and to make sure the safety and stability of residual heat removal of MSRs. Methods： The heat transfers and flow characteristics are analyzed for the cooling thimble of MSRs. An experimental loop is designed and built with a single heat transfer element to simulate the sensible heat and decay heat removal process in drain salt tank when the reactor is shut down. The changes of pressure and temperature are analyzed through which the characteristics of flashing flow instability are studied. Moreover, the changes of heat transfer coefficients are analyzed. Results： The heat transfer rate increases from 550 W to 1 900 W when the thimble outer wall temperature increases from 440 ℃ to 700 ℃. And the heat transfer coefficient changes from 800 W·（m^2·K）^-1 to 1 700 W.（m^2·K）^-1 during the flashing process. Conclusion： The flashing phenomenon will cause the periodical change of natural circulation flow rate and temperature of pipe wall, resulting in the periodic fluctuation of wall temperature and periodic mechanical vibration of the system. In the process of designing the molten salt reactor passive heat removal system, the flashing flow instability phenomenon should be taken into consideration to ensure the safety and stability of heat removal.