Abstract: As multiple levels of electricity markets become intertwined and coupled across different time scales, regional power grids, while serving as hubs for trading channels, also face a series of operational risks, such as network congestion and ensuring security and reliability of supply. In response to the supply security scenario under market-oriented models, based on the analysis of intra-provincial transmission section blocking, an optimization method for inter-regional and inter-provincial power supply capability in the electricity spot market is proposed. Firstly, based on the correlation model of inter-provincial trading channels, inter-provincial physical interconnections, and intra-provincial transmission sections, the mechanism of intra-provincial network congestion formation is analyzed. Secondly, the Gradient Boosting Decision Tree algorithm predicts intra-provincial transmission section blocking, identifying critical sections that form the boundary for electricity purchases in the spot market. Finally, to maximize electricity purchases through inter-regional channels while considering constraints on DC power injection stability, the power supply capability of the target province is optimized. The proposed method is validated by analyzing it based on actual provincial networks.