A surface heat source adjustment approach to calculate temperature field is presented in this paper. In the approach, virtual surface heat source is introduced at the boundary of the object to solve the boundary value problem of the steady-state heat conduction equation. A contact model of rough surfaces with heat conduction connection is established. On this basis, the randomly distributed boundary conditions are established, and the temperature field in the medium is computed by using the surface heat source adjustment method. The result shows that the temperature field presents some random fluctuation due to the untight contact between the actual surfaces. The fluctuation is very obvious near the boundary while it is not obvious at the center, and the smaller the effectively contacted area, the more obvious the fluctuation is. For example, the fluctuation of 50% effective contact is more obvious than that of 75%.