Abstract: The development of the dense wavelength division multiplexing(DWDM) technology requires that the lasing wavelength of the light source have high accuracy and stability.The lasing wavelength stability of the fiber grating external cavity semiconductor lasers(FGESLs) is focused on.After taking into account the joint contribution of semiconductor laser,external cavity and fiber grating(FG) to the phase condition,the mode distribution of the FGESLs can be determined.As a result,the effect of the FG external cavity length on the lasing wavelength of the FGESL is investigated theoretically.Numerical simulations show that for a relative short external cavity,a slight change of the external cavity length will result in an obvious variation of the lasing wavelength;and for a long external cavity(>10cm),the change of the external cavity length gives almost no effect on the lasing wavelength.