It is analyzed that the physical mechanism of decreasing the amplitude fluctuation of output pulse train and eliminating some noise as well as compressing and shaping the pulse train by using a nonlinear optical loop mirror (NOLM). Experimentally the NOLM is placed in an actively harmonic mode locked erbium doped fiber ring laser to form an actively and passively mode locked Er doped fiber laser (APHML EDFL). By means of function of saturable absorber of NOLM, the noise of the fourth rational harmonic mode locked (RHML) pulse train is successfully suppressed and the fourth harmonic mode locked of very stable amplitude with repetition rate of 5.1 GHz is obtained with modulation frequency of 1 GHz.