The micrographs of diesel oil soot particles were achieved by transmission scanning electron microscope with and without water mist applied. It was seen by statistical analysis that the average diameter of soot particle increased and the surface density of soot particle decreased with water mist applied. The experimental results show that water mist washes out smoke mainly by dynamics effect，cloud physics effect and transportation mechanism of Steffen flow. This paper provides scientific references for water mist technology using in smoke scrubbing of computer room fires.