The harmonic characteristics of the AC/DC converters are analyzed based on the harmonic coupled admittance matrix model. The harmonics generating effects of the supply voltage to the converters are composed of three parts， i.e.， the fundamental frequency voltage， the harmonic voltage with the same harmonic order as the harmonic current and the harmonic coupling effect of the converter. Based on the analytical formulas of the matrix model， the amplitudes of the matrix elements are studied. This illustrates the relative importance of the three harmonic generating components. The analytical formula for the typical constant harmonic source model and Norton equivalent circuit model are derived based on the physical significance analysis for the matrix elements. Moreover， a simplified model is proposed which ignores the effects of the harmonic voltage conjugates. Accuracy of the simplified model is evaluated by applying the models into the harmonic power flow calculation. The conclusion can be used as the theoretical foundation for the future harmonic model selection of the converter.