The invention relates to an islanding detection method and device for a direct-current system. The method comprises steps as follows: first power disturbance is injected into a direct-current micro grid, and suspected islanding is diagnosed when the variation of the grid-connected voltage is detected to be higher than or equal to a first admissible value of a voltage difference absolute value within set time or set times; second power disturbance is injected into the direct-current micro grid, and the fact that islanding occurs to the direct-current system is determined when the variation of the grid-connected voltage is detected to be higher than or equal to a second admissible value of the voltage difference absolute value within set time or set times. Accurate protection criteria are provided for off-network of a direct-current transformer, the islanding detection efficiency of the direct-current system is guaranteed, and a non-detection zone is avoided.