The theory of FBG external cavity semiconductor laser with weak feedback is simply explained. Regarding the effects of feedback as a change of photons′ life and using the simple multimodes rate equation, the effect of FBG reflectivity on both the output power and SMSR is revealed.The conclusion is that there exists an optimum FBG reflection. The experiments using FBGs with different reflection show the same conclusion. A single laser mode with 25 dB SMSR is achieved.