To suppress the relative intensity noise (RIN) of fiber DFB lasers, a structure which couples a semiconductor optical amplifier (SOA) into a fiber laser cavity is designed. On the basis of the gain saturation effect of the SOA, 30 dB RIN of the fiber laser at the relaxation oscillation frequency is suppressed. Furthermore, the optical intensity inside the SOA reaches a balanced state via the intracavity SOA structure. Finally, the frequency noise of the laser does not degrade with the addition of the SOA.