Abstract: With the increasing proportion of renewable energy access, the gradual reduction of the effective rotational inertia of the power system and the scarcity of the traditional frequency regulation resources have made the frequency stability of the power grid becoming more and more serious. To quantitatively assess the frequency support capability of the system, on the basis of analyzing the dynamic process of the frequency regulation of the power grid, this paper points out that the frequency support capability should include two parts: the inertia response capability and the primary frequency regulation response capability. Accordingly, the inertia ratio index that may cover the different characteristics of the sending-end and receiving-end power grids is designed and the method for the online assessment of the inertia margin index, the maximum disturbance power index and the inertia ratio index considering the grid operating frequency deviation constraints and the rate of change of the frequency constraints are proposed. Finally, the validity and rationality of the indices and the method are verified with the CEPRI36 calculation example and some actual power grid data.