The interfacial stress in sapphire-GaN heterogeneous film system is simulated using the finite element method herein. The distribution of the interfacial stress and its influencing factors in the process of the growth temperature (1373 K) falling to the room temperature (300 K) are studied. The rationality of the proposed model was verified through theoretical calculations. Results show that the interfacial stress in the system is uniformly distributed along the radial direction, and only the interfacial stress at the edge changes suddenly and drastically. When the thickness of GaN film or sapphire substrate is changed without considering the edge effect while the thickness of another is unchanged, the maximum value of interfacial stress appears when the thickness ratio d_s/d_f of sapphire substrate to GaN film is approximately 1.5, and the minimum value appears when the thickness ratio is approximately 4.3. It shows that the interfacial stress of the film system is not determined by the difference of coefficient of thermal expansion or the difference of temperature, but it is greatly influenced by the thickness ratio. Furthermore, it is found that the stress due to lattice mismatch is much greater than that owing to thermal mismatch.