A loop gravity-assisted heat pipe (LGHP) is characterized by passive heat dissipation and extensive application prospect. Dissimilar working fluids give rise to noticeable differences in their working performance (Critical heat flux (CHF)). Consequently, it is of great significance to evaluate the performance of LGHP working fluid for its design. In accordance with 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. On this basis, the physical property parameter of transitional rough turbulent flow was also derived. Finally, as persuasively illustrated by the experimental results, there was almost a linear association between CHFs and the derived physical parameter. As a result, the physical parameters derived in this paper can be employed as favorable criteria for the selection of LGHP working fluid.