Abstract: The increasing renewable penetration in the power system results in the long-term imbalance of the power system. This paper divides the long-term imbalance risk of the power system into two parts: the continuous low- renewable-output event and the long-term monthly energy supply imbalance risk. First, this paper selects the continuous low-renewable-output scenario and proposes a monthly electricity imbalance risk assessment model based on the Conditional Value at Risk (CVaR) theory. On this basis, a power system planning method considering long-term imbalance risk is proposed. Through the optimal allocation of flexible resources such as seasonal energy storage, the long-term adequacy of the power system could be effectively improved. Finally, the effectiveness of the proposed method is proved based on the case studies on the IEEE RTS-79 system and the role of seasonal energy storage in mitigating long-term imbalance risk is discussed.