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With the development of mobile communication, adaptive algorithms are often chosen in real time process because of their high speed. However, existing adaptive algorithms are not suitable in frequency selective fading channels. In this paper, an improved adaptive synchronization algorithm is proposed for the application under frequency selective fading channels. A novel correlation function is proposed to reduce the effect of intersymbol interference, which only requires coarse channel length estimation. A gate value is also used to combine both the constant and decreasing step size methods; thus, the proposed algorithm is capable of distinguishing between tracking and convergent stages, and then choosing the right step size. Furthermore, an accelerating method is introduced to make the algorithm track faster, i.e. making the algorithm move more steps if it searches in the same direction as the last step. It is shown that the mean square errors of time and frequency offset estimates can be reduced to 0.9243 and 0.003 respectively even when the channel length estimate is not good, which totally satisfies Beek's requirement for synchronization. Simulation also shows that it takes only 10-20 symbols for the proposed algorithm to track the change of channel delay. Compared with the existing adaptive algorithm, the proposed algorithm has a higher accuracy with a higher tracking speed, and the proposed algorithm also has advantages over some nonadaptive algorithms with higher accuracy and lower computational complexity.