Effects of external cavity′s feedback light on length of internal cavity and the photon state density in internal cavity are investigated, and effects of self mixing interference are discussed in detail. The results demostrate that, external cavity′s feedback light not only changes front facet reflectivity of FabryPerot cavity, but also cause discontinuous variation of equivalent length of internal cavity, and then photon state density in internal cavity also discontinuously varies. So that, the external feedback light makes quantum efficiency continuously and discontinuously vary. Also gain coefficient and saturated light intensity vary continuously and discontinuously. Also selfmixing interference signal does not change discontinuously with the complex cavity modes produced by external cavity′s feedback light, and effect of external cavity′s feedback light on charge carrier lifetime is very small. External cavity′s feedback light evidently reduces radiative recombination lifetime of charge carrier, raises radiative recombination efficiency and weakens nonradiative recombination efficiency.