Abstract: The Total Supply Capacity (TSC) represents the operation efficiency and load supply potential of the power grid. Current evaluation on the maximum power supply capacity of the distribution network fails to consider the load transfer after short-term overload of the main transformer, resulting in a conservative evaluation result that hinders the promotion of lean planning and construction of the power grid. In this paper, we propose a method for evaluating the maximum power supply capacity, which takes into account the secondary load transfer. Also, the influence of distributed generation access on the power grid is explored. The load path of secondary transfer is first studied from the perspective of complete load transfer for the faulty main transformer. Secondly, a maximum power supply capacity evaluation model is constructed considering the constraints of main transformer capacity overload and secondary power transfer. Then, the TSC load capacity of each main transformer and feeder in the system before and after incorporating the distributed generation supply is determined by linearizing the model in a modular way. Finally, the influence of 10kV side connection scale on TSC is investigated, and the relationship between the number of substation main transformer stations and the optimal inter-station connection rate is established to enhance the precision of planning. The numerical results demonstrate the effectiveness of the proposed method in evaluating the maximum power supply capacity of the distribution network,excavating the power supply potential, and improving the limitations of traditional evaluation methods. In this way,, the utilization rate of power grid assets is enhanced, which has important guiding significance for accurate power grid planning.