In order to obtain the heat transfer characteristics of the helium gap in conditions of different material thicknesses and linear power in high-temperature ranges, on the basis of previous research, an existent test device was improved. Through the theoretical design of double helium, the test device can perform experiments under high-temperature conditions. Compared with the experimental data, the theoretical design values are in good agreement with the experimental results. According to the design results of the test device, the helium gap test can be successfully carried out out in a high-temperature range, and the test results can provide reference for the design of the material irradiation assembly. The objective of this paper is to set up a multilayer helium gap according to the structural design of the irradiation device to achieve a helium gap test under high-temperature conditions under water circuit conditions.