Based on the theory of Total Supply Capability (TSC), a new method of calculating the User Accessible Capacity (UAC) is given. First, the basic knowledge of TSC theory is introduced, and its mathematical model and solving method is given. Second, the accessible capacity of the feeder 10 kV side of the distribution network can be further obtained on the basis of the results of TSC, as well as the adjustment scheme of the unreasonable feed sections. Third, the known load distribution and its distribution transformer capacity on 0.4 kV lateral is analyzed, and the conversion coefficients of 10 kV side load to 0.4 kV side distribution transformer capacity is got. On the basis of the conversion coefficient, UAC of each feeder is reached conveniently. Finally, a numerical example is taken to prove this method. Determining UAC on the basis of theory of TSC can make full use of the maximum power supply capacity of the distribution network under the premise of ensuring the N-1 security standards, which provides a theoretical basis and auxiliary reference for the power supply enterprise to carry out the business expansion. This work is supported by National Natural Science Foundation of China (No. 51277129).