Abstract: A novel design idea for frequency-locked-loop (FLL) discriminator is proposed, under the premise of changing rate of frequency-rate in returned signal is rather small, the signal after quadrature-demodulation can be regarded as a linear frequency-modulated (LFM) signal, by using estimations of its initial frequency and frequency rate as error control quantities of FLL, FLLs toleration ability to high dynamic signals is greatly enhanced. FFT discriminator and dot-product cross-product discriminator are exploited by FLL discriminator at different phase of frequency-tracking, thus wide discriminate range and high discriminate precision are acquired. An effective and simple abnormal value elimination method is presented especially for the abnormal discriminate value under extremely-low-SNR. Simulation results applying the discriminator to a FLL showed that FLL can tolerate rather high dynamics and tracking accuracy of frequency achieved 0.2Hz even under a SNR as low as -40dB, which verified the validity of above ideas.