In order to research the surface thermal stress in Sapphire/AlN/GaN epilayer and the stress influence factor, the surface stress in materials with diameter of 40 mm were respectively calculated and studied by the finite element modeling method, and the rationality of the model was proved. The dependence between the epilayer surface thermal stress and different parameters were respectively analyzed including growth temperature, AlN transition layer thickness and Sapphire substrate thickness. The results indicated that the epilayer radial stress was an order of magnitude greater than the epilayer axial stress on 1 200 ℃; and the stress was uniform in the radial direction with the variation of ±0.38% in the area with diameter of 32 mm; the epilayer surface thermal stress was in direct proportion to growth temperature during the temperature from 600 ℃ to 1 200 ℃. The results are helpful for study on new technology of epitayers growth and establish the quality choice standard of low stress epilayer.