A study on a tunable fiber laser is presented in this paper. A twenty-meter-long double-cladding Yb3+-doped fiber laser which tuning range is 42 nm in Littman configuration and line width is less than 0.06 nm is pumped by a high-power multimode LD system. The output power is up to 460 mW, and the slope efficiency is about 30%. The optimization of high-power tunable fiber lasers is discussed.