The invention discloses a load flow calculation method and system for a power distribution network. The method and the system are not influenced by high resistance and reactance ratio of the distribution network on the basis that an element module of the distribution network adopts the forward-back substitution method, and have higher computational efficiency and favorable convergence characteristic. During establishing of a layered node array, a distributed power supply model is introduced, so that the method and the system meet the format of the grid to which a distributed power supply is accessed; and in the injection current computation process of nodes, the optimized open-loop model is utilized to simplify the algorithm and improve the algorithm efficiency, so that the method and the system are suitable for various occasions in the distribution network.