A method for designing high porosity composite insulation block was proposed. The numerical simulation of composite insulation block was carried out by using ABAQUS finite element software. The influence of pore configuration， porosity and thermal conductivity of materials on the thermal performance of the composite insulation block was studied. The results show that the thermal resistance of the composite insulation block increases with increase of porosity，decrease of thermal conductivity of the matrix material and filled thermal insulation materials. Through numerical simulation，the relation formulas between the thermal resistance of composite insulation block and porosity, thermal conductivity of the matrix materials, filled thermal insulation materials were obtained. In the case that the mortar joint has little effect on heat transfer of wall, when the porosity of the block is 60%，the heat transfer coefficient of 240mm thickness single block wall can reach 029W/(m2·K)，it can meet the demand of substantial improvement of the thermal performance of block and wall.