Abstract: The high-temperature sodium heat pipe is the key component in the heat pipe reactor. It is used for passive heat transportation. For deeply understanding working media’s evaporation mechanism and characters of heat and mass transfer at liquidgas interface, the molecular dynamics software LAMMPS was adopted to simulate the evaporation of liquid sodium film in 600 K. The mass accommodation coefficient was obtained and its value was 0.388 7. The wall temperature was changed and nonequilibrium simulation was run. The change of liquid film in nonequilibrium were observed, and the net evaporation mass flux and heat transfer coefficient at liquidgas interface were acquired. The results show that after 910 ns, the bottom liquid film thickness, net evaporation mass flux and heat transfer coefficient at liquidgas interface fluctuate repeatedly in 0.10.52 nm, 0.030.07 kg/(m2·s) and 2.23.9 kW/(m2·K), while the net evaporation mass flux at upper liquidgas interface approaches 10-4 magnitude and the heat transfer coefficient is 0.028 kW/(m2·K) and finally decreases to 0.003 5 kW/(m2·K). The results provide reference to numerical simulation of sodium heat pipe in startup phase.