To enhance the service life of magnesia based slag dam, composite slag dam was designed to be cast with alumina magnesia castables in slag line and magnesia castables in molten steel zone. Workability of the magnesia castables for the slag dam was improved and a suitable vibration shaping method was adopted to combine it with alumina magnesia castables. The result shows: (1) workability and setting performance of magnesia castables can be improved to match with alumina magnesia castables by adjusting setting retarder and water reducing agent, and adding proper silica fume; (2) composite slag dam can be prepared with alumina magnesia castables and the improved magnesia castables, whether by up-down composite method or right-left composite method; in order to get full vibration and make interface fluctuation have proper amplitude, the vibration time of the two methods after two different castables contact with each other is 3 and 2.5 minutes, respectively; (3) the result of the on-site test proves that the design aims for reducing pollution to hot metal and improving corrosion resistance have been achieved.