The work aims to prepare a-C:Si coating on silicon carbide substrate and widen the applications of the in high-temperature fields by analyzing its thermal stability mechanism at different annealing temperature. The a-C:Si coating was deposited on the surface of silicon carbide by unbalanced magnetron sputtering, and annealed at different temperature. The coating was characterized and analyzed by XPS, SEM and Raman spectroscopy. The annealing process of a-C:Si coating was simulated by molecular dynamics, and the graphitization behavior of the coating was analyzed from the coating and atomic structure, atomic radial distribution function, coordination number, bond length and bond angle. The thermal stability mechanism of a-C:Si coating was also explored by the cross-analysis of the simulation and experiment data. From the research results, the a-C:Si coating mainly consisted of C and Si elements. Two hybrid bonds of sp2 and sp3 were formed among the carbon atoms. The sp3 bonds were dominant, and their relative content decreased with the increase in the annealing temperature. The Raman curve of a-C:Si coating showed a distinct D peak at 400~500 ℃, and the change trends of ID/IG integrated intensity ratio and G peak value were similar. The simulation results showed that when the annealing temperature increased, the sp3-sp3 bond with a longer bond length in the coating firstly began to transform to the sp2-sp2 bond. As the annealing temperature increased, the sp3-sp3 bond with a shorter bond length began to change. Among them, the sp3-sp3 bond had the highest conversion rate, and Si and C formed the Si-C bond with high thermal stability. The annealing has an important effect on the thermal stability of a-C:Si coating. When the annealing temperature is 400 ℃, the a-C:Si coating begins to graphitize. Si can stabilize the atomic structure, and C-sp3 bonded to Si possesses higher thermal stability, which also reduces the rate of graphitization.