A　one-zone　transient　model　is　developed　for　obstructed　fire　where　extinguishments　primarily　occur　as　a　result　of　reduction　in　oxygen　concentration　due　to　consumption　of　oxygen　by　fire　and　dilution　of　oxygen　with　water　vapor.　The　model　is　based　on　mass　conservation　of　gas　species　and　conservation　of　energy.　This　model　is　used　to　predict　the　fire　extinguishments　by　water　mist　system　under　the　continuous　and　cycling　discharge　modes.　There　is　a　good　agreement　between　numerical　and　experimental　results　in　air　temperature　and　dry　oxygen　concentration.　In　addition,　results　show　that　cycling　discharge　water　mist　system　would　give　more　rapid　extinguishment　and　consume　less　water　for　the　obstructed　fire　in　comparing　with　the　continuous　discharge　mode.