Large-scale separate heat pipe is widely used in various applications, such as waste heat recovery, passive cooling of Spent Fuel Pool (SFP) and so on. The evaporator and condenser of the heat pipe can be installed in different places with long distance. 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. A numerical model of separate heat pipe is established and verified according to the experimental data. The model agrees reasonably well with experimental data. The Mean Absolute Error (MAE) is 7.22%. According to the model, vapor quality at the evaporator exit in the separate heat pipe is analyzed. It is found that, vapor quality varies with heat pipe filling ratio and thermal loads. At low filling ratios, fluid at evaporator exit is overheated. While at high ones, it is in two-phase state. Vapor quality at evaporator exit decreases with the filling ratio. When the vapor quality is bit lower than 1, separate heat pipe comes to its maximum heat transfer capacity. It is also found that there is a minimum condenser height to activate the large-scale separate heat pipe. When the downcomer is full of liquid, elevating the condenser can increase the heat pipe performance. But when the condenser is high enough, the downcomer will be partially liquid filled, elevation of condenser provides little benefit.