Based on the equivalent cavity model,the threshold characteristics of an fiber grating external cavity semiconductor laser(FGECL) are investigated by the multi-parameter optimization of two antireflection and high reflection coatings,and the external-cavity length,and the results of the multi parameter optimization are compared with that of the single-parameter optimization. To obtain the lowest threshold current needs the cost of the reduction of the slope efficiency(namely,the efficiency of electrooptical conversion) and the power of the output light so that the FGECL is of high side-mode-suppression-ratio(SMSR),narrow line-width,excellent dynamic property and high modulation rate. Also it is found that when the reflectivities of two antireflection and high-reflection coatings increase,and the stable oscillation of the master wave is established in the external cavity,the threshold current is the lowest,but the efficiency of the electro-optical conversion and the power of the output light reduce sharply.