A new explanation of phase-conjugation (PC) process in backward stimulated scattering (BSS) is given. There are two nonlinear processes occurring simultaneously in BSS: one is pure stimulated scattering,and the other is partly degenarated four-wave-mixing PC process. The close combination of these two processes makes the BSS to be a PC wave to a certain degree. From this point of view we solved the nonlinear coupled-wave equations for the backward SBS and proved that the backward SBS has a perfect PC property only when some particular conditions are satisfied. A four-wave-mixing detecting method was used to investigate this and the experimental results are in good agreement with our theory.