Abstract: To resolve the problem that the performance of the traditional array invariance method is degraded for ranging a long-distance source with low Signal-to-Noise Ratio(SNR),a Dispersionless Array Invariant Ranging method(D-AIR)is proposed.The dispersionless beamforming is firstly performed to the signals received by a vertical line array,which can eliminate the influence of the dispersion to compress the pulse width and improve the arrival time resolution of normal mode.Then,the angle-time spectrogram output is deconvoluted to improve the elevation angle resolution of the normal mode.The high resolutions of the relative arrival time and the elevation angle in the angle-time spectrogram are beneficial to extract the array invariant and then estimate the distance of the source accurately.The robustness of D-AIR in low SNR is verified by simulation and the Laoshan Bay experimental results show that the relative error of D-AIR is 2.5%less than the traditional array invariance ranging method.