Abstract: As an important part of the electricity trading market, the power sales companies can change the flow distribution of the distribution network by purchasing electricity from the market or delivering electricity to the market and users, thereby affecting the power supply capacity of the distribution network. Previous studies have rarely considered the impact of the uncertainty of power market trading on the operation of distribution networks when analyzing the power supply capacity of distribution networks. Based on the dual role of power sales companies, this paper first analyzes the interaction between market trading and distribution network power supply capacity. Secondly, considering the internal controllable distributed generation and controllable load participating demand-side response within the distribution network, a double-layer optimization model for distribution network power supply capacity considering power sales companies participating in electricity market spot trading is established, and it is solved based on the improved repetitive power flow algorithm and the original dual interior point method. Finally, the improved IEEE 33 test system is used to verify the algorithm, and the results show that power sales company participates in the power transaction by flexibly adjusting the power purchase and sale strategy, which not only ensures the profit but also improves the power supply capacity of the distribution net-work. This research can provide a theoretical basis for optimizing the configuration of the power market and improving the power supply capacity of the distribution network.