Abstract: Several important factors(length of fiber grating,coupling coefficient,reflectivity of internal cavity) which strongly affects the characteristics of fiber grating external cavity semiconductor lasers are analyzed.The analytical theory is confirmed by the following numerical simulations and experimental results.Then the spectrums of FBG-ECL with different reflectivity of fiber gratings are examinated and the perfect spectrum with 0.1nm of bandwidth,power of 417.4μW,37.9dB of side-mode suppression ratio are got.The results demonstrate that theoretical analysis,numerical simulations are in agreement with the experimental observations.