Abstract: Using a helium-xenon mixed gas as the coolant and a closed Brayton cycle as the energy conversion system, the small very-high-temperature gas-cooled reactor with these advantages has become the key research direction for multi-purpose small reactors at present. Natural circulation serves as an important means for energy extraction in small reactors, and its operational characteristics directly affect the safety of the reactor. However, due to the high cost of helium-xenon mixed gas, the analysis of the thermal-hydraulic safety characteristics of the reactor core involving helium-xenon mixed gas is mostly conducted through simulation calculations. The helium-xenon mixed gas experiments conducted mostly focused on the flow and heat transfer characteristics under forced circulation flow conditions, lacking experiments specifically on the natural circulation characteristics of helium-xenon mixed gas. A mixed gas natural circulation flow and heat transfer experimental platform was built, and natural circulation cold start experiments in horizontal heating circular pipe channels were carried out using helium-argon mixed gas with different helium gas volume fractions in this paper. By monitoring the temperatures at different positions of the experimental platform in real time, the time response characteristics of the natural circulation cold start parameters of helium-argon mixed gases with different helium gas volume fractions were analyzed. The results show that the establishment time of natural circulation cold start is positively correlated with the helium gas volume fraction in the helium-argon mixed gas, and has a significant impact. When the helium gas volume fraction is between 20% and 80%, the natural circulation start time is 960 s to 4 100 s (0.27 h to 1.14 h). Under high helium volume fraction conditions, the heating section temperature will first rise to a higher value, then rapidly drop and reach a steady state after the circulation is established. The helium gas volume fraction has little effect on the driving force required for the establishment of the helium-argon mixed gas natural circulation, and the overall driving pressure is between 1 and 2.5 Pa. But the driving pressure head after stabilization is negatively correlated with the helium gas volume fraction, and the driving pressure head is between 8 and 19 Pa. The helium-argon mixed gas natural circulation shows a significant thermal stratification in the heating section, and the temperature differential between the upper and lower fluids at the outlet section of the heating section in the experimental conditions reaches a maximum of 113 ℃. The experimental results can provide a reference for subsequent helium-xenon mixed gas natural circulation experiments.