The invention discloses a harmonic analysis method for a power system. The method comprises steps: power system signals are acquired, the power system signals are uniformly sampled, and discrete signals are obtained; cubic spline interpolation fitting processing is carried out on the discrete signals, and a continuous function after fitting is obtained; and non-uniform sampling is carried out on the continuous function after fitting, arithmetic Fourier transform is carried out on sampling data, and according to the obtained signal frequency spectrum after transform and the data, harmonic analysis processing is carried out. The cubic spline interpolation algorithm is adopted for function fitting, the precision is higher, the errors are small, and the power system harmonic analysis accuracy can be ensured. The invention also discloses a harmonic analysis device for the power system, which does not need to be described.