Abstract: The higher-order theory was developed to three-dimension,coefficients of temperature function were replaced by surface-averaged temperature to be primary variables of equations,and the concept of sub-cells was canceled.The number of equations was reduced so that computational efficiency was increased effectively.During the thermal analysis of represent volume element,heat exchange equations which were established by the relationship between hear flux and average temperature were used to compute temperature,The comparison of numerical results with finite element analysis demonstrates the correctness and validity of this method.