Based on the ray tracing method,considering the gain distribution with wavelength,the expressions of the output spectrum and output power of the fiber grating external cavity semiconductor lasers (FGECL) are deduced firstly. Combining with carrier rate equation,the fine structure of output spectrum and output power are numerically simulated. The results show that,in the reflective band width of grating the multi-peak structure appears,and the output spectrum becomes firmer with the decrease of the front facet reflectivity. The P-I characteristic of FGECL has been investigated. With the decrease of the front facet reflectivity,the fluctuation of the output power tends minimized and the P-I curve becomes generally a straight line.