Anti-islanding approach with current perturbation of particular frequency in dc microgrids has attracted wide attention in recent years. Nevertheless, the static perturbation frequency selected from the rated conditions shifts with system power variation, and thus will lead to a detection failure. In this article, the function of current perturbation to the output voltage of distributed generations is derived to illustrate the root cause of the frequency shift problem. On top of that, the design method of detection threshold is proposed to ensure successful detection. At last, the effectiveness and accuracy of the detection threshold design method is proved by the simulations.