The existing online estimation of primary frequency modulation ability does not consider the effect of the dead zone on the primary frequency modulation of power grid. In order to characterize the effect of frequency modulation dead zone on the primary frequency modulation, the power grid primary frequency modulation capability (Primary Frequency Modulation Ability, PFMA) index is proposed by the means of square deviation. The static state and dynamic state expressions are given according to the primary frequency modulation model. Finally, the evaluation index curves without dead zone and under different dead zones are calculated by simulation. The results show that the dynamic characteristic and dead zone of primary frequency modulation capability are considered, and a comprehensive evaluation of primary frequency modulation capability is realized.