Electroabsorption modulator (EAM) has been known with high transmission efficiency, saturation absorber, the high nonlinearity and high speed. In this work, we use a high-speed EAM in a fiber ring structure to generate short pulse train by active mode-locking method. Inside the cavity, Erbium Doped Fiber Amplifier (EDFA) is adopted as the gain medium due to its wide operation wavelength range (about 1530nm to 1570nm) and high gain saturation power. The mode-locked optical pulse train driven by a 4 GHz microwave source has been successfully designed and measured with filter-limited 21.2 ps pulse width (FWHM) and output power of 10 mw. Clean and stable level of signal with fundamental frequency of 4GHz is obtained through the measurement by optical spectrum analyzer, electronic spectrum analyzer and sampling scope. By tuning the optical path, the highest optical transmission with the shortest pulse and the largest photocurrent is found in EAM, indicating the dominated mechanism is the saturation of optical absorption.