A complete and precise static modal of fiber grating external cavity semiconductor laser is improved based on the coupled-cavity laser model. By taking consideration of the coupling efficiency between the laser diode and the fiber grating, the coupling coefficient η1 from the laser to the fiber grating and the coupling coefficient η2 from the fiber grating to the laser diode are both involved in the elements of scattering matrix describing the coupled-cavity model, and the analytical expression of the coupled-cavity model is modified. It is found that the product of the coupling coefficient η1 and η2 affects the gain curve. Additionally, analysis indicates that shorter external cavity and shorter fiber grating length determine the single lasing mode oscillating in the lower region of the gain curve. Diode core and fiber facets reflectivity also play important roles in the threshold current and side mode suppression ratio (SMSR), especially in the case of relatively high reflectivity. This can be solved by carefully designing the length of air gap, resulting in the match of the external cavity modes and Fabry-Perot (F-P) modes of diode. Accordingly the optimized design principle is put forward by general considering of the parameters.