Abstract: A coordinated decision-making method for provincial power companies in multistage and multi-time scale markets is proposed. Firstly, the optimization space of annual and monthly power purchase is identified based on the medium and long term power balance. Secondly, on the basis of the existence of power purchase optimization space, based on the two-latitude risk analysis method, the power purchase optimization of provincial power companies considering the risk and cost of power purchase is carried out. Taking the minimum power purchase cost as the goal and adding the risk constraint, the optimization model considering both the minimum power purchase cost and the minimum risk is constructed. Finally, according to the construction of supporting DC power supply in Qinghai province and the current situation and development of Henan power grid, taking Qinghai-Henan ±800 kV UHVDC transmission project as an example, the power purchase strategy of Henan Electric Power Company in 2020-2025 after Qinghai DC is put into operation is optimized and the feasibility and effectiveness of the proposed method are verified.