Abstract: The mortised modular energy-saving wall is a new heat-insulating system, which can save energy efficiently by holding temperature in the gap between two wallboards. Joint together by mortises and tenons, various inner partition and nonbearing walls can be provided in a modular way. Considered the wall as two-dimension heterogeneity construction enclosure, the average thermal resistance was calculated. Based on the theories of stable heat-transfer of siding, heat-transfer coefficient is discussed, as well as the economical thickness of insulating layer of the new-type wallboard. Calculation and analysis indicated that the heat-transfer coefficient of the new-type wall is 0.689 W/(m²·K) only, far less than that of any other walls. And it means that the new type wall is highly energy-saving. When the polystyrene board thickness is 0.08 m, the wall is most economical with a heat-transfer coefficient down to 0.400 W/(m²·K).