The theory model of fiber Bragg grating external cavity semiconductor lasers(FBG-ECL) is studied based on mode coupled theory and principle of lasers firstly,the analytic solution of fiber grating reflectivity is given in this paper.Using rate equations,the high frequency response characteristic of FBG-ECL is taken into account.Utilizing equivalent external cavity approximation model,the influence of coupling coefficient on the threshold gain of FBG-ECL is discussed.Adopting best peak reflectivity will make FBG-ECL having high output power and high fine sidemode suppression ratio.Lastly,the output spectrum of FBG-ECL in different peak reflectivity are obtained practically measured.