High-penetration renewable integration induces grid inertia degradation, exacerbating frequency instability. The inertial support strategy of traditional energy storage systems (ESS) may lead to serious damage to energy storage or an overload of inverter equipment. To overcome these drawbacks, a variable coefficient inertia support strategy is proposed for ESS applications. Firstly, the frequency link of the distribution network is integrated into the control of ESS voltage. Then, a variable coefficient inertia support strategy is designed to dynamically adjust the voltage-frequency coupling relationship, which considers the state of charge of ESS and the available capacity of the inverter. Furthermore, a power correction mechanism driven by quadratic voltage control is adopted to adjust the reference power. Finally, an active-reactive power coordination controller is established to maintain AC voltage stability during inertia support. Compared to traditional virtual inertia methods, the maximum frequency deviation during load switching is reduced by 69.75%, and the time required to reach the lowest frequency point is significantly decreased.