The characteristics of the backward scattering phenomena generated by the 81 cm-1 bandwidth KrF laser pumping SF6 medium are explored. The spot patterns of the backward scattering light does not change with the pumping light spot, but its size is enlarged with the increase of the focal length of the lens. There exists maximum for the pulse width and the energy of the scattering light, which rise and then fall, with the rise of the gas pressure of the medium. The energy reflectivity is 1%～3%, and there exists pulse width compression effect for the backward scattering light, related with gas pressure of the medium. With the focusing lens and the medium pool replacing the back mirror, auto cavity-forming and laser output are realized.