With　4th-order　Runge-kutta　method　and　relaxation　method,　we　obtain　the　power　distribution　of　pump　light　and　signal　light　along　fiber　by　solving　the　steady-state　rate　equations　in　ytterbium-doped　double-cladding　fiber　amplifier.　The　relationships　between　the　output　power　and　the　fiber　length　are　discussed　with　different　pump　power　and　different　doped　concentrations　of　the　fiber.　The　change　of　the　output　power　versus　the　pump　power　is　calculated.　The　power　distribution　along　fiber　and　the　relationship　between　output　power　and　input　power　are　analyzed　when　the　signal　is　injected　from　z=L　and　z=0　respectively.