In this paper, the relationship between correlation interval (CI) and estimate is investigated. Then a special correlation interval is explored that is adaptive to all levels of signal-to-noise ratio (SNR) and velocity conditions, and the mean square error is deduced. Finally, we propose an iterative algorithm that achieves the special correlation interval and calculates the Doppler spread by increasing the resolution on time-domain iteratively. Simulation results show that compared with conventional schemes, performance of the proposed algorithm is basically independent of velocity variation and less sensitive to SNR, especially in low SNR environments. It achieves high accurate estimation directly without any post-rectification.