To enhance the coordination between the capacitor energy of modular multilevel converters (MMC) and system frequency regulation during primary frequency control, an online frequency optimal control strategy for distribution networks based on MMC capacitor energy support is proposed. First, based on a frequency response model, the relationship between the frequency nadir and the intervention frequency of capacitor energy is analyzed. The frequency nadir is estimated online through power-frequency characteristic fitting, and an optimal intervention frequency is determined by considering the margins for capacitor energy release and absorption as well as reliability constraints. Then, a global coordinated control strategy for converter stations is proposed. By improving the traditional droop characteristics, the capacitor energy of non-disturbed converters is coordinated to provide frequency support without requiring real-time communication. Finally, an AC/DC distribution network model is built in MATLAB/Simulink to verify the effectiveness and feasibility of the proposed control method.