Using a mathematical model of heat transfer of unattached underground engineering envelope, the calculation area was divided into 14 rectangular blocks according to the interzone temperature profile estimation (ITPE) technology, and the solutions were obtained for all the parts using the technique of variable separation. The Fourier coefficients are determined based on the continuity of the heat flux and boundary conditions, as a result, a system of linear equation group including 26N equations has been obtained. By changing the parameters, the influence of heat characteristics of envelope building material and soil, temperature of earth surface and heat transfer coefficient between air and wall on heat course of the envelope has been quantitatively analyzed.