In the process of water mist fire suppression the phenomenon of fire expansion may be found initially. Experiments and theoretic study show that evaporation of mist on the surface of fuel plays a crucial role in the fire expansion. In experiments of varied conditions, it is found that volatility of fuel and characteristics of water mist determine the degree of fire intensification. It is found that water in the bottom of tray will make fuel surface temperature and fire heat release rate descend. And with increase of water quantity, fire intensification will be weakened, extinguishing time also be shortened. So further research on change of fire heat release rate owing to water pouring is necessary to ensure the reliability of experiment data.