In fire environments，the traditional fire extinguishing agents can't effectually put out a fire which is sheltered by barrier. Now, a lot of water mist fire extinguishing researches have been made . This paper mainly studies the validity of water mist fire extinguishing and investigates the controlling and extinguishing effects of water mist fire extinguishing when barrier exists. At the same time, this paper also studies how the key factors, including opening degree between nozzle and barrier,water mist pressure, water mist flux and so on, influence the fire extinguishing validity, analyzes the water mist fire extinguishing characters when barrier exists, gives the reason why the water mist can't put out a fire in some conditions，and provides references for practical engineering application.