In a low-noise supersonic wind tunnel at a Mach number 3.4, visualization of flow structure around backward facing step (BFS) with a 3 mm high step is carried out via schlieren and nano-tracer-based planar laser scattering (NPLS) respectively. The time-averaged flow characteristic of the reattachment region and the rich instantaneous structures of the redeveloping boundary layer are both revealed. By contrasting the NPLS images at different times, the unsteady evolvement characteristic of the coherent vortices in the redeveloping boundary layer is discussed. And the results are compared with the schlieren of Mach 4.2 and the prior data published. Results indicate that with either of the two flow visualization ways, the shock waves and the expansion waves can be captured; however, the NPLS technique has the obvious advantages to reveal the instantaneous structures on a small scale in a certain section plane with a time resolution of 6 ns and spatial resolution about micron magnitude; under the flow condition in this contribution, the growth rate of redeveloping boundary layer is 0.07519; the characteristic time is around 10 μs of the hairpin vortex shedding. At the same expansion rate, the reattachment occurs later with increasing Mach numbers, while if the expansion rate increases, the reattachment occurs earlier, and the flow turn angle is larger.