A water cooling structure of a reducer and a reducer assembly are disclosed. The water cooling structure comprises a chamber formed by a reducer housing and a cover plate, and the cover plate is fixedly connected to the chamber. The chamber is provided with a water inlet and a water outlet respectively. The chamber is also provided with one or several partition plates on two opposite side walls. The partition plates are arranged in an interdigitating manner and each of the partition plates is connected with only one side wall of the chamber, and there is a gap between the partition plates and the opposite other side wall of the chamber, so as to form an S-shaped water path. The water inlet and water outlet are respectively disposed at both ends of the water path. A plurality of baffles are further vertically provided on the partition plates and side walls of the chamber that are parallel to the partition plates, and the baffles are arranged in an interdigitating manner. The water cooling structure disclosed in the present disclosure is integrated with the reducer housing into one part, and thus has a simple structure, saves space and is convenient to arrange on the vehicle. Moreover, the cooling efficiency is further improved by the above special structure.