Loop gravity-assisted heat pipe (LGHP) has the characteristics of passive heat dissipation and wide application prospect. Different working fluids lead to great differences in their working performance (Critical heat flux (CHF)). Therefore, it is of great significance to evaluate the performance of LGHP working fluid for its design. Based on the basic theories of CHF and hydrodynamics, the pressure drop models of laminar, smooth turbulence, and completely rough turbulence flows were established, and an extensible two-phase composite property parameter was obtained. Based on it, the physical property parameter of transitional rough turbulent flow was also derived. Finally, experiments were conducted and showed that there was almost a linear relationship between CHFs and the derived physical parameter. Therefore, the physical parameters derived in this paper can be used as good criteria for the selection of LGHP working fluid.