A widely tunable Er3+/Yb3+ co-doped fiber laser with simple structure is demonstrated. Using a blazed grating as the wavelength selection element, and a semiconductor laser diode as the pumping source, stable tunable output laser in 1550 nm wave-band is obtained with large-mode-area Er3+/Yb3+ co-doped double-cladding fiber as the gain medium. The tuning range is 36 nm, covering almost the whole fluorescence spectrum. The linewidth is less than 0.08 nm for every tuned laser. For different wavelength, the output power is different, and the output power is higher than 400 mW within 25 nm tuning range. The maximum achieved power is 510 mW with the wavelength of 1543.86 nm and the calculated slope efficiency is 26%. With wide tunable range and narrow linewidth, this stable efficient laser can be used in dense wavelength division multiplexed (DWDM) optical fiber communication systems and high resolution fiber sensor systems.