The utility model relates to the construction field of subway stations, in particular to a rail top air duct in a subway station, and solves the problems of long construction period, high construction risk, complicated construction technology, much difficulty in controlling the construction quality and the like, which are caused when a conventional subway rail top air duct adopts a sheet iron air pipe and reinforced concrete air duct for construction. The rail top air duct provided by the utility model is formed by assembling a plurality of sections of prefabricated air duct units prefabricated by concrete, the adjacent prefabricated air duct units are connected through bell and spigots, so that the construction of median plates is slightly influenced, the construction precision is easy to control and the construction quality is greatly improved; the phenomena that the rail top air duct construction, the air duct and the like forcibly occupy the effective construction period of a station main structure are avoided, so that the construction period is greatly shortened; the prefabricated rail top air duct can be mounted without the need of suspended aloft work, so that the safety factor is higher; and the process is simple, the prefabricated air duct units adopt combined steel mold industrial operation, the construction quality is controllable, the installation of embedded parts and the formation of prepared holes are simple and the prefabricated air duct has the advantages of high precision, high consistency, high production efficiency and the like.