The invention relates to a BPA data based alternating current system harmonic impedance scanning method. The method is characterized by comprising the following steps: (1) establishing each element harmonic impedance model in an electrical power system; (2) reading BPA data, and inputting an appointed frequency range and observation nodes; (3) carrying out node numbering optimization by adopting a semi-dynamic method based on an improved wiring matrix; (4) forming a node admittance matrix line by line according to an optimized numbering sequence; (5) solving a node voltage equation; and (6) changing frequency in the appointed frequency range according to a frequency scanning range, reforming a node admittance matrix, and returning the step (5) to solve the node voltage equation so as to obtain harmonic impedance under different frequencies. Compared with the prior art, the computation speed is obviously increased and is not restricted by node quantity and node voltage level, and the method is widely applied to harmonic impedance computation in the electrical power system.