Harmonic power flow calculation is the basis of harmonic analysis, tracing, and treatment. This paper focuses on the harmonic issues associated with the implementation of the “dual-carbon” plan in novel urban distribution networks. Due to the widespread integration of renewable energy generation into urban electrical grids, an increasing number of power electronic devices are being connected to the grid, which has a certain impact on harmonic power flow. Firstly, the harmonic components of 6-pulse and 12-pulse rectifiers are analyzed for the switching power supply model widely used in the distribution network. Secondly, the equivalent parameter models of the generator, the transformer, and the lines are reviewed. Finally, from the perspective of power balance, a harmonic power flow model considering the terminal switching power supplies and the correlation between impedance and frequency is proposed. Simulation verification is conducted to demonstrate the feasibility and accuracy of the proposed method.