Digital filter is one of the key technologies that affect the quality of marine gravity data processing results. A real time positive and negative finite impulse response(FIR) filtering algorithm is proposed and the expression of the positive and negative FIR filter with linear phase is derived. The filter designed by this algorithm satisfies the strict linear phase-frequency characteristics in all sampling spectrums, and will not cause nonlinear distortion to the signal; The low-pass filter designed by the algorithm has a large stop-band attenuation, which is beneficial to suppress high-frequency noise in ocean gravity data. The ship-borne test shows that the filtering effect of the positive and negative FIR filter is better than that of the traditional FIR filter in ocean gravity data processing, and comparable to offline processing results.