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A full-scale passive residual heat removal system for 2 MW molten salt reactor has been designed and constructed to perform experimental studies. The present research aims at investigating the transient behaviors of natural circulation during the start-up process, as well as steady state characteristics of heat transport capacity and temperature distribution in the drain tank. It is seen that natural circulation will not initiate in the loop until boiling occurs in the heating section, which is resulted from the special structure of cooling thimble. In case of single-phase natural circulation in the cooling thimble, theoretical equations for predicting the flow rate are derived based on considerations of flow regime variation and heat exchange inside the loop. Six cooling thimbles have been used in the system. At normal operation temperature of 643 °C, each cooling thimble has a heat carrying capacity of 2665 W. It is found that for heat transfer from thimble tube to bayonet tube, radiation and conduction heat transfer dominate in steady state conditions and start-up transient, respectively. In addition, temperature distributions inside the drain tank suggest that molten salt may start to freeze even though the bulk temperature is much higher than the solidifying point.