Variational principles of smoke temperature are studied in confined space with water mist applying.  Effect of ventilation area,  position between ventilation and fire and water mist flux on temperature decreasing velocity is studied by experiments.  The attenuation of temperature decreasing velocity is described under different ventilation conditions.  It is found in experiments that the attenuation of temperature decreasing velocity caused by ventilation can be reduced by increasing water mist flux.  This paper provides necessary references for water mist application in fire protection.