In order to study the form mechanism of Laser Supported Detonation (LSD) in Laser Propulsion, an experiment was made. YAG laser 2J/pulse with pulse - width of 5 nsec, focalized through the convex lens and exploded around the focus. Using the Hi-spec processor and schlieren apparatus in the experiment the emission images of laser sustained plasma and schlieren images of disturbed air field were obtained, and then the duration time of plasma and other characteristic parameters were calculated. By changing the material parameters, the plasma wave's magnitude increased obviously, the duration time lengthened the threshold value of LSD fell distinctly.