A digital orthogonal decomposition method for satellite navigation anti-jammer antennas is proposed in this paper based on FIR filters. Firstly, the responses of pass-band, stop-bands and the number of filter orders are determined according to the intermediate frequency and bandwidth of the satellite navigation signals. Then, an odd symmetrical FIR filter and an even symmetrical FIR filter are designed by using the Parks-McClellan algorithm. Finally, I channel signals and Q channel signals can be got by filtering the real intermediate frequency output from the AD converters using the odd symmetrical FIR filter and the even symmetrical FIR filter respectively. The method proposed in this paper has less computation load and suits realization in projects. The testing results verify the effectiveness and practicability of the method in this paper.