Abstract: In order to study the effect of cooling gas on the temperature distribution around the capsule and the hohlraum, CFD simulation was conducted to obtain the maximum temperature gradient on the deuterium-tritium capsule surface and the gas flow field when the hohlraum filled in different helium content gaseous mixtures under different gas pressures. In addition, the variations of the average temperature and the maximum temperature gradient on the capsule surface were recorded under the condition that the cooling walls were applied to thermal oscillation. The results indicate that increasing the gas pressure or decreasing the helium content enhances the natural convection difference between the upper and lower gas regions, which results in the worse temperature gradient on the capsule surface. While, decreasing the helium content in the cooling gas which gets lower thermal conductivity and higher specific heat reduces the influence of cooling wall temperature fluctuating on the temperature distribution around capsule.