Passive detection method has a larger blind zone, and active frequency drift (AFD) method can reduce blind zone while the power quality will be lowered in islanding detection of distributed generation system. This paper proposes an islanding detection method based on blind-zone estimation. By determining whether load equivalent resistance is operating in an active power non-detection-zone state or not, the AFD method will be introduced and passive and active detection method will be combined to judge the islanding. The proposed method can realize non-blind zone detection and meanwhile reduce the harmonic injected frequency of AFD method and lower the influence of distributed generation system on output power quality. Matlab simulation shows that the algorithm is reliable, and the grid-connected inverter can detect the islanding fast and effectively.