Abstract: A simulation model of water mist is established to study the fire extinguishment mechanism in case of a fire under the micro-gravity environment. Atomizing characteristics of the water mist are investigated under different gravity environments by the FDS. It is found that the spray momentum is reduced under the micro-gravity environment, but the scope of protection is increased. Numerical simulations of the fire being suppressed by the water mist in a typical manned spacecraft are carried out, and the temperature and the oxygen concentration against time are obtained. It turns out that the water mist can suppress the fire by flame cooling and oxygen dilution effectively.