A lower module of a power electronic device includes a lower module included in a power electronic device that is divided into an upper module and a lower module by an intermediate plate and comprises a blower fan to introduce air into the inner space of the lower module; a plurality of capacitors installed in a suspended form on the intermediate plate and spaced apart from the blower fan; a heat sink having a plurality of heat dissipation fins through which heat generated from a heating component disposed on the upper module is conducted, and disposed adjacent to the capacitors to cool the plurality of heat dissipation fins by air moved from the plurality of capacitors; a DC reactor cooled by the air moved from the heat dissipation fins; and a discharge plate having a vent hole to discharge the air moved from the DC reactor to the outside.