Based on the fact that the system frequency control method in micro grid is much difficult than the situation in traditional distribution power grid as the micro grid contains a variety of DERs, this paper researches on the control method to know how much the shortage gap of the active power storage in the abnormal frequency state during the off-grid operation of the micro grid system. A fast calculation method of the active power vacancy is proposed and the meaning of this method for the off-grid micro grid frequency stability is analyzed. Numerical example result shows that, this method for the active power vacancy can be used not only to guide the frequency stability control system adjusting the active power of the equipment in the off-grid micro grid accurately in order to maintain the system frequency within the range of conformity, but also to guide simulation verification of the control strategy about the frequency stability of the micro grid.