Phase-shift grating is a key component which forms the resonance cavity of DFB fiber laser. Due to additional loss, coupling coefficient and phase-shift grating length distinctly affect the optimum output power of DFB fiber laser. These factors were investigated by transfer matrix method. Through numerical simulation, the variation of the optimum output power with the optimum coupling coefficient and grating length under conditions of different cavity loss was analyzed. It shows that the optimal output power can be obtained by adjusting the optimum grating length and coupling coefficient according to the level of cavity loss. These conclusions can provide guidance for the optimized structure in designing the fiber laser.