Power electronic converters are single input multiple output (SIMO) systems in frequency domain. In distributed power system (DPS), one converter's switching frequency ripples are another converter's perturbations, and beat frequency components are generated due to their SIMO characteristics. As the control loops of power converters always take high gains in low-frequency regions if the beat frequency components are relatively low, they may be magnified and presented as oscillations, which will introduce system instability. This paper proposes a matrix-based multifrequency output impedance model to describe the SIMO characteristics of power converters and to analyze the stability of DPS. The buck converter is specifically illustrated as demonstration. The proposed model indicates that the variation of switching frequency, as well as those traditionally discussed parameters of control loops and passive components, could also change output impedance characteristics and introduce system instability. In DPS, the beat frequency oscillation that traditional models fail to explain could be accurately predicted by the proposed model. In addition, based on the proposed model, the switching frequencies for power converters can be optimized to improve the stability of power electronic-based systems. Simulation and experimental results validate the accuracy and the effectiveness of the proposed method.