Nowadays phase change materials (PCMs) have been used in preliminary practical engineering. However, the selection of the thermal design parameter of PCMs is really difficult on account of the change of specific heat capacity, thermal conductivity in phase change region and the impact of the latent heat. Thermal conductivity can't be used in the selection of PCMs. To provide guidance in thermal design of the wall, relative thermal conductivity (RTC) of the PCM layer was defined and its computing method was given. RTC is a thermal-physical parameter that can represent the same heat transfer capacity of PCMs with that of normal materials in the same conditions. Based on the computing method, RTC of the PCM layer in different areas were calculated, and heat transfer characteristics of PCM layers in the whole year were shown. In addition, PCM layers' utilization effects in different areas of China were analyzed. By comparing different PCM layers' RTC, it was obtained that different layers had different utilization effects. RTC can be used to select the proper PCM used in a certain area.