Interaction between water mist and fire smoke is studied by experiments in ISO9705 room. The variational principles of oxygen, carbon dioxide and carbon monoxide concentration is disclosed and the mathematics relations of smoke component concentration with water mist pressure and ventilation speed are established according to the experimental results, it is found in the experiment that when ventilation speed exceeds 1.5kg/s the smoke component concentration breaks. This paper provides necessarily theory for water mist technology using in smoke restraining.