A tunable Yb3+-doped double-clad fiber (YDCF) laser has been demonstrated experimentally using a high-power 975 nm laser diode (LD) as the pump source, a large-mode-area YDCF as the gain medium and a blaze grating-based Littrow configuration as the tunable mechanism. With the optimal fiber length of 14 m and the effective reflectivity 20% of the blaze grating, the threshold pump power of the laser is ～1.3 W. Through rotating the blaze grating, the output lasing wavelength can be broadly tuned from 1046 nm to 1121 nm. When the injected pump power is 48 W, a tunable output of 23.7 W at 1089 nm is achieved with a slope efficiency of 53%. Numerical simulation is performed to analyze the laser output characteristics, and the experimental results are in good agreement with those by numerical simulation.