The invention discloses a device and method for system inertia during isolated running of a microgrid, which is used for solving the problem of difficulty in effectively suppressing frequency fluctuation of a power grid during isolated running of an existing microgrid. The device comprises a microgrid (1) and a DC power supply (5), wherein the DC power supply (5) is connected with one end of an AC reactor (3) by an inverter (4), the other end of the AC reactor (3) is connected with the microgrid (1) by a circuit breaker (2), a controllable DC voltage converter (6) is connected in parallel to two ends of the DC power supply (5), and a capacitor (7) is connected in parallel to an output end of the DC voltage converter (6). By the device, an effect of energy buffer on the microgrid is achieved, and the effect of preventing power grid frequency from substantially fluctuating is achieved.