The release of the power supply side increases the uncertainty of the power flow of a distribution network, and also has a certain impact on the power supply capacity of the network. In this paper, the factors that affect the power supply capacity are analyzed from two aspects:physical factors and market factors. Secondly, according to the change of power transaction load and the load growth trend simulated by least squares method, the power supply capacity model of a distribution network considering the release of the power side is established, and an improved repetitive power flow algorithm is used to solve the model. Finally, the influence of factors such as the increase of tie line and the size of electricity transactions on distribution network power supply capability is analyzed and the accuracy and effectiveness of the proposed method are verified by an example. This work is supported by National Key Research and Development Program of China (No. 2018YFB0905105) and Science and Technology Project of China Southern Power Grid in 2019 “Assessment Method Study of Distribution Network Power Supply Capability after the release of the Power Supply Side” (No. ZBKJXM20180068).