During mixture of Nitrogen and spray water ejecting quenching under normal pressure and high velocity, the liquid film that is formed on the surface of specimen to reduce the heat transfer between specimen and quenching media is removed on the one hand; on the other hand, the heat transfer performance of the mixture exceeds that of pure Nitrogen. Because the surface heat transfer coefficient is difficult to measure, according the cooling curve of surface and centre of specimen measured experimentally, the law of surface heat transfer coefficient and specimen temperature is calculated by nonlinear estimate methods and finite difference method based on inverse heat transfer method. The results show that the cooling performance of mixture of Nitrogen and spray water is as well as that of water or oil. During quenching, the surface heat transfer coefficient increases rapidly at begin, and at temperature of 170 °C, the surface heat transfer coefficient decreases. During martensite phase transformation, the latent heat is used to increase drive force of phase transformation and to overcome resistance of phase transformation, thus the martensite phase transformation can fulfill.