In the power system, to analysis the harmonic effect of a concerned harmonic source load, it is need to establish an equivalent system in access point for the network without the concerned load. The equivalent system can be expressed as the harmonic voltage source and the harmonic impedance of the series. In this paper, a method is proposed to calculate the parameters of the equivalent system such as the equivalent harmonic voltage and the equivalent harmonic impedance. The method makes every five sampling points as a set to linear regression based on complex field to achieve the harmonic voltage and harmonic impedance parameters of the equivalent system, and then build the single-point equivalent model of grid. The accuracy of the proposed method is verified by the IEEE14-bus system simulation based on PSCAD/EMTDC software.