By introducing the rotatable in-line polarizer into the ring cavity of nolinear polarization rotatation(NPR), the all fiber structure of passively mode-locked ring laser is simplified. The Erbium-doped mode-locked laser with the tuning range of 20 nm is achieved by adjusting the polarizer, 1/2 wave plate and the 1/4 wave plate together. Furthermore, the mode-locked pulses with dual-wavelength are observed during the tuning process. On the other hand, when removing the 1/2 wave plate from the configuration, the mode-locked laser can also be obtained by only adjusting the polarizer and the 1/4 wave plate. The tunable process is presented and analyzed.