The invention provides a design and manufacturing method of a slag skimmer of an iron runner of a blast furnace with a single tap hole. An over beam of the slag skimmer is formed by using a refractory castable for performing, and other parts (an outer upper of the slag skimmer) of the slag skimmer are formed by using the refractory castable for on-site casting. The position for arranging the preformed over beam is reserved in the outer upper. The over beam is then mounted in the reserved position. The castable or a ramming material is used for filling gaps on the periphery of the over beam. The slag skimmer produced according to the scheme can realize long service life and be replaced quickly.