As an important part of the electricity trading market, the power sales company is an important part of the electricity trading market. It can not only be used as the main body to buy electricity from the market transaction but also as the main body to transmit electricity to the market and users, which changes the power flow distribution of the distribution network and has a certain impact on the power supply capacity of the distribution network. Based on the dual roles of power sales companies, this paper, first, analyzes the interaction between market transactions and power supply capacity of distribution networks. Second, considering the internal controllable distributed generation and controllable load participating demand-side response, a double-layer model of distribution network power supply capacity considering the participation of power sales companies in spot trading is established. According to the characteristics of the model, this paper uses the improved repetitive power flow algorithm and the original dual interior point method to solve the model. Finally, the improved IEEE33 node is verified. The example shows that the 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 network.INDEX TERMS Power sales company, electric power transaction, power supply capacity, improved repetitive power flow algorithm.