Experimental investigations into deflagration of approximate - quiescent state and pre-mixed combustrial vapor cloud were conducted. The overpressure distributions of hemispheric mixtured vapor clouds with diameters 1

2 and 3m respectively were experimentally measured. The pressure time histories (p-t curves) at different positions were given. The characteristics of deflagration overpressure were discussed. The results can be used to verify theoretical methods and provide reference for further research.