Abstract: The toroidal field (TF) and poloidal field (PF) coils are the key components of the International Thermonuclear Experimental Reactor (ITER). To verify whether the performance of the manufactured TF and PF coils meets the performance standards, low-temperature superconducting performance tests need to be conducted in a low-temperature environment before installation. According to the structure and testing requirements of the TF and PF coils, a large-scale low-temperature testing device was designed. Based on the structural design of the testing device, multi-layer insulation (MLI) as thermal was used to shield the thermal load from the external environment. Combined with the insulation performance requirements of the coils for the testing device and the actual installation conditions, the materials, lap joint treatment, and structure of the MLI were designed. Through thermodynamic analysis and calculation, the optimal number of MLI layers was determined, and the heat leakage and temperature of each layer of the MLI were numerically calculated using MATLAB software, providing data references for subsequent optimization. Finally, the theoretical analysis and numerical simulation of variable density multilayer insulation (VD-MLI) are carried out by the orthogonal experiment method. The thermal insulation performance and heat leakage trend of VD-MLI under different density configurations are compared, which provides guidance for thermal shield design of low temperature test platform.