Ultra-short optical pulse sources are key components in optical
time-division multiplexing(OTDM) system. In this paper, a wavelength and repetition frequency tunable actively mode-locked fiber ring laser based on cross-gain modulation in a Reflective Semiconductor Optical Amplifier(RSOA) is demonstrated. Theoretical model for this scheme is also established and output characteristics are calculated. Stable picosecond pulse trains at 10-80 GHz repetition frequency are obtained. 30nm wavelength tuning range is achieved in our proposed actively mode-locked laser.