The problem of LFM pulsed radar's ranging bias due to the range-Doppler coupling is studied. The mechanism of the ranging bias is derived and analyzed. The nolinear residue of the bias neglected by most researchers is pointed out emphatically. And the relationship between this residue and the parameters of the radar waveforms and target motion is analyzed. An unconstrained optimization algorithms is proposed for the estimation of the bias residue. At last, experimental results prove the analytic conclusion and estimation algorithm are correct.