The economic insulation thickness is a thermal design method that considers both energy conservation and economy. The existing researches have often ignored the effects of solar radiation. Taking a typical three-story external thermal insulation building in Xi’an as an example, this paper introduces solar radiation through the OTTV algorithm to calculate the relationship between insulation thickness and life cycle cost of the external wall of the building. A life cycle cost analysis model was established to determine the economic insulation thickness of the external insulation building in Xi’an. The paper uses the theory of non-equilibrium heat preservation to calculate the thickness of economic insulation in different orientations and provides new ideas and methods for the study of economic insulation thickness.