The variation of casting hot spot with proceeding of solidification and components of casting-mold system is studied by the technique of numerical simulation of solidification. The result shows that the thickest part of casting is not exactly the last part of solidification in the casting, while the last part of solidification is not exactly casting hot spot at the early stage of solidification. The location, size, shape and number of casting hot spot change with geomitric, physical and technological factors of the casting-mold system such as thickness of the casting secondary wall and with the passage of time in the course of the solidification. The former is known as the systematic property of hot spot and the latter, dynamic property. Only when the properties of hot spot are grasped completely and accurately, can it be fed more effectively. By doing so, not only sound castings can be obtained, but also riser efficiency can be improved. The variation of casting hot spot with proceeding of solidification and components of casting-mold system is studied by the technique of numerical simulation of solidification. The result shows that the thickest part of casting is not exactly the last part of solidification in the casting, while the last part of solidification is not exactly casting hot spot at the early stage of solidification. The location, size, shape and number of casting hot spot change with geomitric, physical and technological factors of the casting-mold system such as thickness of the casting secondary wall and with the passage of time in the course of the solidification. The former is known as the systematic property of hot spot and the latter, dynamic property. Only when the properties of hot spot are grasped completely and accurately, can it be fed more doing. By doing so, not only sound castings can be obtained, but also riser efficiency can be improved.