Planar laser induced hydroxyl fluorescence (OH-PLIF) technology has been used to image the hydroxyl of supersonic combustion supplied by the hydrogen fuel being ejected to the mach 1.72 freestream in the three different streamwise sections. Employing a frequency-doubled dye laser with wavelength of 283.553nm as exciting laser，the fluorescence intensity of hydroxyl is nearly indirect proportion to the mole fraction of hydroxyl. The highly turbulent flame structure of supersonic combustion has been revealed from the instantaneous PLIF images of hydroxyl. The steady combustion in the cavity can offer on-duty flame stimulating and stabilizing the supersonic combustion. The flame spreads in the radial direction as well as in the stream direction. Having bigger spreading scale of the cavity, the streamwise flame region would diminish a little.