In order to solve the problem that there are many criterion in the traditional passive isolated island detection method based on frequency and phase and the switching process is long in the actual flexible HVDC system, based on the actual situation of flexible DC transmission system, a d-axis reference current islanding detection method for AC/DC hybrid power grid is proposed. In this method, the output d-axis reference current of the outer loop controller is used as the criterion, and the threshold setting method is given according to the characteristics of the outer loop controller, which changes obviously under the condition of isolated island and can effectively avoid misjudgment. Finally, an AC/DC hybrid connecting simulation model is built to verify the effectiveness, accuracy and convenience of the isolated island detection method proposed in this paper.