Abstract: In the new power system, the imbalance of the supply and demand for adequacy and flexibility intensifies in different space-time. Optimizing the HVDC transmission plan allows optimal scheduling of adequacy and flexibility resources in DC interconnected power systems. First, the DC daily trading electricity quantity is obtained by decomposing the cross-region medium-and long-term trading electricity quantity considering the balanced allocation of adequacy over time and implementation deviation of the HVDC day-ahead transmission plan. Then, a supply and demand model for adequacy and flexibility is established. On this basis, an optimization model combined with the decomposition of medium-and long-term trading electricity quantity for the HVDC day-ahead transmission plan is proposed based on the Stackelberg game. The leader optimizes the HVDC transmission plan to promote the spatiotemporal equilibrium of supply and demand for adequacy and flexibility between the sending and receiving ends. The follower performs unit commitment and economic scheduling through two-stage stochastic programming. Finally, the model's effectiveness in cross-regional optimal scheduling is verified on the wind power HVDC transmission system.