Abstract: Due to the ultrawideband characteristics of underwater acoustic (UWA) channels, the Doppler frequency offsets of different subcarriers are not the same in UWA orthogonal frequency division multiplexing (OFDM) communications. The accuracy of Doppler scale estimation suffers from the influence of this non-uniformity. In this paper, the frequencies of null subcarriers are calculated according to the expected Doppler scales. And then the cost function which is the powers of all null subcarriers versus the expected Doppler scales is achieved by non-uniform fast Fourier transform (NUFFT). A Doppler estimation algorithm of searching the minimum position of the cost function by the method of steepest descent is proposed. This algorithm improves the precision of Doppler scale estimation by avoiding the error of estimating non-uniform carrier frequency offsets (CFO) of different null subcarriers. Meanwhile, the computation complexity of calculating the powers of all null subcarriers is reduced by NUFFT. Finally, a shallow water experiment was conduct to demonstrate the performance of the proposed method. Excellent performance results are obtained in ultrawideband underwater acoustic channels with a relative bandwidth of 67%, when the transmitter and the receiver are moving at a relative speed of 5kn, which validate the method proposed.