The binary-phase-coded echo signal is modulated by both Doppler and the echo phase displacement.Aiming at this problem,we analyzed the echo model and the power distribution characteristic of the echo signal of radar altimeter,and proposed a demodulation algorithm for the binary-phase-coded echo signal based on nadir point.In the algorithm,the Doppler and the phase displacement information of the nadir point echo was detected,and corresponding compensation was made to realize demodulation.Simulation and experiment results show that the maximum error of the Doppler frequency is less than 20 Hz and the maximum error of the phase is less than 2°.It proves that the algorithm is effective to demodulate the echo signal.