Large tonnage machinery has the characteristics of being bulky, expensive and complicated to operate, with the bad production environment where it is inconvenient to study and operate such equipment. In view of this situation, this paper develops a computer simulation technology as the core of the 3D simulation training system in large tonnage machinery. It can be applied to large tonnage machinery operator training. The key issues are introduced. The 3D simulation training system in forging machines developed with Java 3D technology is illustrated as an example. In the applications, the 3D simulation training system of large tonnage machinery is safe, practical and economical