In order to improve the output power and processing quality in industrial applications, it is important to optimize the output characteristics of the high-power fiber lasers. The slope efficiency, backward leaking power and stimulated Raman scattering are key issues in high-power fiber laser design. The parameters matching the fiber gratings, which are the critical components, have a direct influence on the whole fiber laser system. In this paper, the parameters matching the fiber gratings in fiber lasers are investigated. Firstly, the origin of slope efficiency, backward leaking power and stimulated Raman scattering are analyzed in theory. Then the influences on output characteristics of fiber lasers comprised of the output coupler gratings, which have different bandwidths and reflectivities, are experimentally studied. Finally, the optimized parameters and matching principle of fiber gratings in high-power fiber laser are obtained , thus providing an alternative method to improve the output characteristics of high-power continuous wave fiber laser.