Abstract: In existing schemes of cold neutron sources, two-phase thermo-siphon loop is widely adopted for its high heat exchange performance. However, the void fraction in moderator may affect the stability of moderation. In this paper, the single-phase thermo-siphon loop was studied in its performance of dealing with high heat flux in order to replace the two-phase one. A couple method based on iteration, which combines the one-dimensional theoretical formula and three-dimensional numerical simulation, was presented to analyze the feasibility of single-phase moderator cell scheme in CARR cold neutron source. Both macro-performance of the loop and the detailed information such as flow and temperature field inside moderator cell can be analyzed by this method. The results show that the thermo-siphon loop only takes away about 30% of the total heat release. However, with the addition of helium cooling jacket, the rest of heat can be removed by the direct heat transfer between the metal walls and helium gas. The temperature field shows maximum temperatures of liquid hydrogen and walls are 21.7 K and 23.7 K, respectively. The above results indicate that the moderator cell can be cooled adequately, which proves the single-phase thermo-siphon with helium direct cooling as well as the corresponding moderator cell scheme can meet the requirement of CARR project.