Abstract: The cone top cover, as one of the critical pressure boundaries of the fast reactor primary vessel, is crucial for the safe operation of the fast reactor. The structural integrity of the cone top cover is closely related to the temperature load, which is significantly influenced by the coupled heat transfer characteristics of natural convection and thermal radiation in the argon space. It is necessary to conduct in-depth research. A scaled experimental platform, with a 1∶5 scale, of the heat transfer characteristics of the argon space above the sodium liquid level was designed and constructed. Based on this platform, experimental research on the coupled heat transfer characteristics of natural convection and thermal radiation in the argon space was conducted. The temperature distribution of the cone top cover, argon space, and main pump support under different sodium liquid level conditions was measured to elucidate the influence of the sodium liquid level on the heat transfer characteristics. The experimental results show that the temperature distribution along the dimensionless radius direction of the cone top cover may exhibit various patterns such as increase, plateau, or first increase and then decrease, which are related to the intensity of natural convection heat transfer, radiation heat transfer, and the geometric characteristics at different positions. The temperature of the cone top cover increases with the increase of the sodium liquid level, and due to the proportionality of thermal radiation to the fourth power of thermodynamic temperature, the influence of the sodium liquid level on the temperature of the cone top cover is more significant at high temperatures. Furthermore, the overall temperature of the argon space increases with the increase of the sodium liquid level, but only increases significantly near the sodium liquid level, and the temperature gradient along the height direction of the gas cavity is very small. This experimental result indicates the presence of a layer of sodium vapor near the sodium liquid level, which absorbs thermal radiation. The experiment also finds that the temperature of the main pump support increases overall with the increase of the sodium liquid level. Within the range of experimental parameters conducted, the influence of the sodium liquid level on the circumferential temperature difference of the main pump support is very weak. By conducting research on the effect of the sodium liquid level on the heat transfer law in the argon space, it can provide a basis for establishing the temperature mapping relationship between the experimental device and the prototype reactor and provide a reference for the design and safe operation of sodium-cooled fast reactors in our country.