Insulation is one of the most effective measures in energy-saving renovation of existing buildings. The determination of type and thickness for the insulation material is crucial in the energy-saving design of wall, which is determined by multiple target parameters. An optimization model combined the numerical method with multi-objective genetic algorithm (NSGA-Ⅱ) is established based on the inverse problem research method. The total energy consumption of the unit area wall and the transformation cost are selected as the objective parameters, and the synergic design for type and thickness of the insulation material is made for the existing building wall renovation in Chengdu. The results show that there are 6 of the 8 kinds of insulation materials are not the optimal solutions with the conventional thickness. However, the solutions can approach the Pareto frontier by changing the thickness or unit price of the insulation materials, and a balance can be realized between the minimum annual energy consumption and reconstruction cost. The method proposed offers a new idea for building energy-saving design. The conclusions can provide references for the similar energy-saving renovation in Chengdu.