By means of the technique of rational harmonic mode-locking(RHML), we succeeded in obtaining the optical pulses at repetition rate (fp) being integer multiples of the RF drive frequency(fm≈1.4 GHz) with detuning ratio of﹥10-6. The generation of maximum frequency was about 6 GHz and the stable rational harmonic mode-locking occurs only at fp=2fm. According to the result of our experiments, we put forth a possible mechanism of using RHML to generate high bit-zate pulses and point out the cause of the occurrence of instability in higher PHML. Furthermore, we demonstrate an effective method to elliminate the amplitude fluctuations of output pulse train.