Steam-air decoking process on the inner surface of cracking furnace tube is a dynamic process with the variation of temperature, pressure, concentration and coke layer thickness along the reactor length. Assuming that the kinetic parameters and reaction heat of the decoking reaction along furnace tube remain unchanged, the mechanism model of steam-air decoking process was established based on the analysis of steam-air decoking mechanism. According to the actual decoking operation of the cracking furnace BA104 at SINOPEC Zhenhai Refining & Chemical Company, the first principle model was derived and solved with finite element analysis method. The comparison of industrial simulation results with actual production verified the feasibility and effectiveness of the derived steam-air decoking model, laying foundation for the closed-loop optimal control of decoking process.