The passive residual heat removal system of molten salt reactor consists of a boiling and a condensing loop, which are connected by a steam drum. A numerical model based on two-fluid model in RELAP5 is developed to demonstrate the use of the passive system and investigate the interaction between the boiling loop and the condensing loop at steady state. The numerical work is partially validated by experimental results obtained in a single-tube natural circulation loop. Heat flux, initial water inventory, the height of the condensing loop and the heat transfer capacity of the condenser are changed separately to study how the system responds. The results show that the mass flow rate in the boiling loop indeed has a maximum. Both the maximum value and the system pressure are mainly affected by heat flux and heat transfer capacity of the condenser. The mass flow rate in the condensing loop is controlled by heat flux instead of other parameters, thus it seems that the condensing loop is a follower of the boiling loop. The initial water inventory and the height of the condensing loop have little influence on the system performance. Additionally, Bubbly flow, slug flow, vertical stratified flow, horizontal stratified flow and annular-mist flow appear in the natural circulation system.