Lithium heat pipes have broad applications in heat pipe cooling reactors and hypersonic vehicles owing to their ultra-high working temperature. In particular, when the length of the lithium heat pipe is ultra-long, the flow and heat transfer characteristics are more complex. In this study, an improved lumped parameter model that considers the Marangoni effect, bending effect, and different vapor flow patterns and Mach numbers was developed. Thereafter, the proposed model was verified using the University of New Mexico’s Heat Pipe and HTPIPE models. Finally,the verified model was applied to simulate the steady-state operation of an ultra-long lithium heat pipe in a Heat PipeSegmented Thermoelectric Module Converters space reactor. Based on the results:（1） Vapor thermal resistance was dominant at low heating power and decreased with increasing heating power. The vapor flow inside the heat pipe developed from the laminar to the turbulent phase,whereas the liquid phase in the heat pipe was always laminar.（2） The vapor pressure drop caused by bending was approximately 22–23% of the total, and the bending effect on the liquid pressure drop could be ignored.（3） The Marangoni effect reduced the capillary limit by hindering the liquid reflux, especially at low vapor temperatures.Without considering the Marangoni effect, the capillary limit of the lithium heat pipe was overestimated by 9%when the vapor temperature was 1400 K.（4） The total thermal resistance of the heat pipe significantly increased with increasing adiabatic length when the vapor temperature was low. Further, the wick dryness increased with increasing adiabatic length at any vapor temperature. Such findings improve on current knowledge for the optimal design and safety analysis of a heat pipe reactor, which adopts ultra-long lithium heat pipes.