In this paper,  at first research of relation between correlation-interval and estimation accuracy is carried out,  and the optimal correlation-interval is proved to be existent that is adaptive to signal-to-noise ratio and velocity varieties. Then some special characteristics of the optimal correlation-interval are explored. Based on them an iterative correlation-interval optimized (CIO) algorithm is proposed.  It optimizes the correlation- interval and calculates the Doppler spread by increasing the resolution on time-domain iteratively.  Simulation results show that contrasted to conventional schemes the proposed approach is less sensitive to signal-to-noise ratio and velocity varieties,  and can improves the performance dramatically in low signal-to-noise ratio environment in which the conventional schemes has a poor performance. It achieves the high accurate estimation of Doppler spread directly.