Active frequency drift（AFD） method is one of the most popular islanding detection methods. The paper analyses the principles， advantages and disadvantages of the traditional AFD method. Based on these works， an improved scheme of detecting frequency variations first and then determining the value of chopping fraction is presented. The performance indices of the improved method are in accordance with the standard of IEEE Std 929—2000， meeting the requirements of power systems in China. Simulation results on Simulink show that the improved method can detect the islanding operation effectively with faster detecting speed， smaller non-detection zone， and less harmonics pollution to the grid.