In order to evaluate the energy efficiency of phase change energy storage composite materials in building applications, the concept of relative thermal conductivity was proposed from the perspective of energy, based on properties of phase change materials. And the testing method, i.e. energy compensation method for implementing the concept was also proposed. A self developed device was made use of to test reference board, ordinary plasterboard, expanded perlite composite board and phase change plasterboard for thermal conductivity of thermal insulation（reference board）. Results show that the proposed method can not only test the relative thermal conductivity of phase change energy storage composite materials, but also traditional insulation materials. And it can be used to achieve the thermal performance evaluation and provide technical support for phase change energy storage composite materials in building applications.