For practical interferometric fiber-optic sensor applications, various kinds of noise sources exist and become one of the most significant factors that deteriorate the system performance. In this paper, aiming to suppress the noise, a new low-pass-filter (LPF)-assisted rectangular-pulse binary (RPB) phase modulation method is proposed. The LPF is applied directly in the light intensity domain, but interestingly when combined with the RPB method, this can lead to very strong resistance ability to the noise in the whole frequency range as will be proved in this paper. The low-frequency-filtering is accomplished by the RPB method naturally during the phase-demodulation process. The high-frequency-filtering is accomplished by using a low-pass filter with carefully chosen cut-off frequency. Both simulations and experiments are conducted to validate the effectiveness of the new modulation method, and significant signal-to-noise ratio (SNR) improvement (7.7 dB in the experiment) is obtained.