The existing methods and models for calculating and plotting the power supply capacity curve of distribution network do not accurately take the network loss into account, and the results are difficult to apply to the actual distribution network. In order to solve this problem, a new BSC model based on power flow calculation method and considering network loss is proposed. Secondly, a sampling algorithm and pointwise approximation are proposed to adapt to the calculation of network loss and obtain a complete set of points describing the security boundary. Finally, the errors between the proposed method and the existing literature methods are described by comparing the examples. The results show that the BSC curve in this paper is more accurate, which provides a reference for the application of BSC theory from planning to operation.