In view of the existing phase-difference algorithm's dramatic performance degradation in instantaneous frequency estimation for Linear Frequency Modulation (LFM) signals when Signal-Noise Ratio (SNR) is low,an improved algorithm is proposed based on N-order phase-difference.Firstly,based on phase reconstruction,the adaptive incremental threshold is set up to confirm the frequency ambiguity position for the frequency mutation caused by SNR threshold,and the loop iteration is utilized for interpolation correction.Then,instantaneous frequency is estimated by high-order phase difference by phase jump information,and the iterative reweighed least square method is used for linear fitting.Finally,the idea of self-adaption is introduced to deal with input signals with different frequencies,especially with low frequency.Simulation results show that,compared with existing phase-difference method,the proposed algorithm can keep low computational complexity while improving anti-noise performance,and the lower limit of input SNR requirement reduces by 4 dB approximately.