The fire extinguishment time is a major factor to evaluate the efficiency of fire extinguishment with water mist. In this paper the fire extinguishment time with water mist in an enclosed room is calculated. Before adding water mist, the chemical kinetics plays the role in combustion, where a dimensionless math model is established by us-ing the Semenov theory. After adding water mist, the diffusion plays the role instead. Then another math model containing water mist and dominated by oxygen concentration is established. The fire temperature is integrated from Tm to extinguishment temperature TB and the extinguishment time can be obtained. The calculated values are compared with the experimental data under different conditions. The results show that this model can predict the fire extinguishment time accurately. Besides, this model also can be used to determine the critical water mist flux and evaluate which fire extinguishment mechanisms dominate the extinguishment. The fire extinguishment time is a major factor to evaluate the efficiency of fire extinguishment with water mist. In this paper the fire extinguishment time with water mist in an enclosed room is calculated. where a dimensionless math model is established by us-ing the Semenov theory. After adding water mist, the diffusion plays the role instead. The results show that this model can predict the fire extinguishment time accurately. Besides, this model values ​​can be compared with the experimental data under different conditions. water mist flux and evaluate which fire extinguishment mechanisms dominate the extinguishment.