Supposing that input modulation signal is actual phonetic signal and its adjoint noise is additive white noise, for spontaneous emission factor modulation, by using small signal appoximatation, the signal-to-noise ratio (SNR) gain is derived in various parameters through frequency domain analysis against laser in the premise of large input SNR The results of numerical simulation show that when the pass band range of the band pass filter (BPF) is 300-3400 Hz, signal-to-noise ratio gain oscillates with bias current in the background of constant 1; while the pass band range of BPF increases to a certain degree, tunning-up modulating bias current and parameters of the cavity can improve the anti-noise properties of laser.