An approach to passive impulsive source range estimation in shallow water is proposed. The approach is based on warping transformation of the energy density function of the received signal. Because of the influence of the sea bottom, it is difficult to find a warping operator adapted to the dispersion characteristics of modes in shallow water. Even though the modes can be separated from each other by using the warping operator adapted to the ideal waveguide, it is impossible to obtain the analytic solutions of the characteristic frequencies, while the energy density function of the received signal is not affected by the sea bottom. Like the received signal and the autocorrelation function of the signal, the frequency spectrum of the warped energy density function of the received signal also owns invariable frequency features. These characteristic frequencies equal the difference in the cut-off frequency between two modes in ideal waveguide, which are easy to calculate with the knowledge of the depth and the average sound speed of the water. What is more, the warping operator transforms mode pairs in energy density function with the same mode number difference into one monotone, which means one characteristic frequency is not unique for one mode pair. In shallow water, the acoustic field is typically composed of a group of modes with close mode numbers. Therefore, the smaller the mode number difference, the more the mode pairs, and the higher the spectral peak of the corresponding monotone is. When the source range is unknown, the approximate relation formula between the extracted characteristic frequency in a supposed source range and the real characteristic frequency is derived, based on which a fast passive source range estimation method is proposed. The proposed method successfully avoids using the guide source and the calculation of replica field, which is necessary in existing passive range estimation algorithms. And applying warping operator to the energy density function of the received signal makes it easy to obtain the analytic solutions of the characteristic frequencies, which is impossible in previous researches. The method is successfully applied to the Yellow Sea impulsive signal data collected by a single hydrophone in November 2011. The mean relative error of range estimation is less than 8%.