Abstract Commutation failure is one of the most common faults in high voltage direct current (HVDC) transmission system. Subsequent commutation failures (SCFs) caused by commutation failure will cause large voltage and current swing. Several SCFs may cause DC transmission power interruption, which seriously threatens the safe and stable operation of AC/DC hybrid power grid. With the construction and commissioning of grid-side electrochemical energy storage (EES), it is possible to mitigate SCFs of adjacent HVDC transmission lines using EES with fast power response characteristics. In this paper, the influence mechanism of active and reactive power output of EES on commutation conditions is studied by combining the evolution of cascading outages and SCFs. It is found that reactive power mainly affects AC voltage and active power mainly affects DC current. Based on the mechanism analysis, a coordinated power control strategy for EES is presented. This strategy, combined with EES capacity constraints, can control EES active and reactive power output instructions adaptively according to the degree of AC failure at different stages of SCFs to fully release the transient support capacity of EES. Case studies are undertaken on the Henan multi-infeed UHVDC system and CIGRE-HVDC standard test system, respectively. The simulation results verify the correctness of the proposed EES power control strategy and the effectiveness of the strategy for SCFs mitigation in different systems, different fault types, and different fault degrees.