Abstract: The superconducting cavity vertical test Dewar is a liquid helium open vessel, which has high requirement for thermal insulation performance. The inner vessel of the Dewar is insulated with high-vacuum multilayer insulation, and the top cover side is multi-screen insulation. In order to reduce the leakage heat on the top cover side of the Dewar, a variable density radiation shield method was proposed. The coupling heat transfer model of heat conduction, convection and radiation was established for the cover, radiation shield, liquid surface and helium gas, and was verified by experiment. The influence of the number of radiation shield on the leakage heat and the optimal radiation shield density were obtained by numerical calculation. The result shows that the calculated result of the radiation shield temperature is consistent with the experimental result, with an average relative deviation of 8.37%, and the heat transfer model is considered to be reasonable. The Grashof number (Gr) of the helium gas between the first radiation shield (shield 1) near the liquid surface and the liquid surface has a maximum value (T₁=9.14 K, Gr=1.12×10¹⁴) with the increase of temperature T₁ of shield 1, and after T₁ exceeds 35 K, Gr remains basically unchanged. When the radiation shield is arranged at equal space, the total leakage heat change is not obvious after the number of radiation shield is greater than 11. Under certain number of radiation shield, the gas heat conduction between two adjacent shields is dominant, and the heat conduction transfer decreases and the radiation heat flux increases from the low temperature area close to the liquid surface to the high temperature area near the cover. As the number of radiation shield is 11, the method of the variable density radiation shield, which is to be arranged for 7 radiation shields in the high temperature area and 4 radiation shields in the low temperature area, has minimal leakage heat. The method can reduce the heat leakage by 4% compared with the equal space method.