The invention provides a regional power grid scale and power supply capacity assessment method, and includes the steps of: selecting a mode of a transformer station for supplying power to a medium voltage side power grid; through high/medium voltage side bus short-circuit current interaction and coordinated calculation, determining allowable access medium voltage side power grid generator capacity of a supply area; and according to transformer capacity and the allowable access medium voltage side power grid generator capacity of the supply area, determining comprehensive power supply capacity of the supply area. The invention provides a calculation method for calculating allowable access medium voltage side power supply generator capacity and comprehensive power supply capacity of a subarea grid, has good adaptation, can rapidly determine the allowable access unit capacity and comprehensive power supply capacity of the subarea grid, remarkably reduces a calculation workload, and improves calculation efficiency.