There are now many intermittent renewable energy sources such as wind power and PV being connected to the power system. This occasions a problem of system frequency stability caused by system inertia reduction which becomes increasingly prominent, and it is necessary to consider the influence of system inertia in flexible resource allocation. For system flexibility analysis, this paper introduces the characteristics of net load after a high proportion of renewable energy is connected and offers a calculation method of system inertia based on system generator inertia and capacity. Then, a calculation method for the lowest inertia of the system is proposed based on the maximum frequency change rate and maximum power change. Finally, combining with the process of flexibility analysis, a method to examine the minimum inertia constraint in flexibility analysis is proposed. This method is applied in the process of flexible resource optimization. This can change the flexible resource allocation in the low inertia state of the system, improve the inertia and the frequency stability of the system, and realize the combination of system flexibility and frequency characteristics. This work is supported by the National Natural Science Foundation of China “Multi-Energy Collaboration based Distributed Renewable Energy High-Proportion Consumption and Efficient Utilization” (No. 51961135101).