Abstract: s: One of the important characteristics of functional languages is polymorphic typestwhich are very powerful and are one of the important mechanisms used to implement polymorphism in object-oriented programming languages. However, the implementation of polymorphictypes is quite different from that of the basic types and other compound types. This paper givesthe important data structures of implementing polymorphic the uqies analyzes the type inference ofSML functional language compiler and reveals the essence and implementation techniques relatedto polymorphic types.