By using a tunable-ratio optical coupler (TROC) to adjust the wavelength dependent intra-cavity loss, a L-band mode-locked erbium-doped fiber-ring laser (ML-EDFL) is demonstrated for generating wavelength-tunable femtosecond pulses. The change of output coupling ratio introduces different intra-cavity loss and shifts the peak of mode-locked gain profile to provide continuous detuning on wavelength of the ML-EDFL. A maximum tuning range of about 40 nm (from 1565.1 to 1605.3 nm) by decreasing the output coupling ratio from 95% to 5% is obtained, corresponding to a wavelength tuning slope of 2.25 nm/dB. The ML-EDFL exhibits a super-mode suppressing ratio as high as 47 dB and a pulsewidth of <5 ps at repetition frequency of 1 GHz. Nearly transform-limited pulsewidth of 580 fs is generated by linear dispersion compressing the EDFL pulses with a 32.5m-long single-mode fiber under an output coupling ratio of 10%.