A cooling system for a power conversion device may include a cooler upper-part having a plurality of cooling tubes through which cooling water flows and a connecting portion connecting the cooling tubes; one or more power conversion modules mounted between the cooling tubes; a cooling fin plate formed in a plate shape with cooling fins on a side and mounted on a side of the power conversion module such that the cooling fins are mounted in the opposite directions with respect to the power conversion module; and a cooler lower-part mounted between the cooling fin plate and the cooler upper-part, having one or more open holes formed at a portion, and combined with the cooling fin plate by inserting the cooling fins of the cooling fin plate in the open holes.