Abstract: In order to control accurately the lasing wavelength of the fiber grating external cavity semiconductor laser(FGECSL),after determining the mode distribution based on the phase condition,the influence of the air gap between the head of fiber and the front face of F-P LD on the lasing wavelength of FGECSL was through numerical simulation.The results show that the variation of the air gap will result in the lasing wavelength rippling around the Bragg wavelength of fiber grating(FG),and the amplitude of fluctuation becomes smaller with the increase of the external cavity length.For a fixed length of external cavity,the lasing wavelength can be accurately located in the Bragg wavelength of FG through adjusting the air gap carefully.