Offshore heavy oil reservoirs are abundant, with broad development prospects. However, the environment of fire flooding is harsh, so it is difficult to ensure safe exploitation. In order to study the integrity of cementing system, based on the Mohr-Coulomb criterion, mechanical theory and temperature field distribution, the first interface micro-annular of cementing was calculated. Considering the effects of non-uniform in-situ stress, downhole high temperature, and formation temperature on the casing-cement ring-formation combination system, and obtain the distribution diagram of the first interface micro-annulus. The results show that downhole temperature, elastic modulus of cement ring, Poisson's ratio of cement ring, thermal expansion coefficient of cement ring, and thermal conductivity of cement ring all have an impact on the first interface micro-annular of cementing, and the safety of cementing device can be improved by adjusting the parameters of cement stone.