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There is significant negative impact due to an unintended islanding supported in DG systems，resulting in developing passive and active methods to detect islands. Many active techniques have been proposed and AFD method is classical. However，the method has large non-detection zone and the chopping fraction may extend the run-on time and fail to detect islanding operation. The improved method proposed in this paper monitors the frequency tendency first and then decides the suitable chopping fraction，ensuring fast and effective antiislanding operation. The standard of the method based on IEEE Std. 929-2000 fits the power condition of China. The simulation results show that the proposed algorithm is capable of detecting the islanding condition correctly，with smaller non-detection zone and less harmonics.