Abstract: To enhance cold performances of thermal insulation layers in large LNG storage tanks, it is necessary to optimize the thickness of such thermal insulation layers. With combination of theoretical analysis and numerical calculation, cold performances of insulation layers with different thicknesses are assessed in respect of cold losses, vaporization rates, and stress and deformation of outer tank walls. Then, optimal thickness of the insulation layer is determined. Following conclusions are drawn from the study. Firstly, solar radiation should not be neglected in the cold performance analysis and related design of the insulation layer because it has a great influence on the temperature field of storage tank. Secondly, thickness of the insulation layer (mainly expanded perlite) is inversely proportional to the cold loss rate. When the thickness of the expanded perlite reaches a certain value, the cold loss rates decrease slowly as the thicknesses increase. Thirdly, changes in thicknesses of the thermal insulation layers will affect stresses and deformation of the outer tank walls. Consequently, cares shall be taken for impacts of insulation layers with different thicknesses on structural safety of tank walls.