Impedance　is　very　important　for　power　electronic　systems　because　of　the　close　relationship　between　impedance　and　system　stability.　The　power　converters　are　typical　nonlinear　dynamic　systems　in　frequency　domain.　With　a　sinusoidal　input　voltage　perturbation　excitation,　the　input　current　contains　not　only　the　input　perturbation　frequency,　but　also　multiple　other　frequency　components.　Therefore,　the　input　impedance　has　single　input　multiple　output　(SIMO)　characteristic　in　frequency　domain.　However,　traditional　impedance　modeling　methods　only　take　the　component　at　input　perturbation　into　consideration　and　simplify　the　model　in　a　single　input　single　output　(SISO)　form.　They　are　actually　simplifications　of　real　input　impedance　in　low　frequency　situation,　which　fail　to　explain　some　oscillations　in　distributed　power　system　and　micro-grid.　This　paper　proposes　a　new　input　impedance　model　to　describe　its　SIMO　characteristic.　The　buck　converter　is　introduced　specifically　as　demonstration.　The　proposed　model　could　accurately　explain　the　oscillations　in　a　system　consist　of　two　cascaded　power　electronic　converters,　whereas　traditional　models　fail　to　do　so.　The　simulation　results　validate　the　accuracy　and　effectiveness　of　the　proposed　model　and　obtained　conclusions.