A multi-spectral line interpolation harmonic analysis method and system based on main lobe width relate to the field of harmonic analysis. The method comprises the steps of preprocessing signals; determining spectral line number; determining weight; calculating a correction formula of a multi-spectral line interpolation algorithm; calculating fundamental wave parameters; determining harmonic parameters; and performing error analysis. A frequency domain expression after windowing of power grid signal (a current signal or a voltage signal) of a power system serves as a starting point, the numberof the spectral lines participating in operation is determined for the first time according to the main lobe width of the windowing function, and the magnitude of each spectral line weight is deduced. Harmonic analysis is carried out on such basis, and harmonic correlation parameters are calculated. It is proved through calculation that an interpolation analysis method carrying out by utilizing all spectral lines in the main lobe is high in precision, high-precision measurement of harmonic parameters is achieved, and good use value is ensured.