© 2020 Elsevier Ltd A separate heat pipe is a kind of wickless heat pipe in which flow boiling occurs in the evaporator and flow condensation occurs in condenser. The evaporator and condenser can be installed in different places. Working fluid circulates in the heat pipe due to static pressure difference between the evaporator and condenser. The separate heat pipe is a high efficiency heat transfer device which can transport heat for a long distance without considerable losses. It is becoming more and more widely used in various fields, such as heat recovery, solar energy collection, air conditioning, passive cooling and so on. In the present study, a numerical model for a separate heat pipe is established and verified with experimental results. Reasonable agreement with MAE of 3.94% between the model and experimental data is obtained. Further studies depict that vapor quality at evaporator exit varies from 0.1 to 1 and shows great self-adjustment ability under different working conditions. Due to the self-adjustment, the separate heat pipe can maintain its heat transfer performance in the charge range of 0.55 m–1.35 m. And this range is strongly affected by both vapor quality at evaporator exit and liquid column height in the downcomer.