This paper reports the temperature relaxation process at melting iron/sapphire interface under shock compression, and interpretes the phenomenon basing on transformation kinetics and heat conduction equation. Analysis shows that if iron keeps melting, the history of interface temperature is affected by the melting speed. The interface temperature undergoes a slow cooling process when the melting speed of iron is finite, which is different from the result of previous researches. Simulation signal can be made accord ant with the experimental signal by adjusting some parameters. It was found that the melting temperature and melting speed of iron at high pressure can be dctermined by researching the relaxation process. So, this paper shows a new theoretical method and its experimental verification.