Abstract: This paper discusses the design and implementation of ASN.1 (Syntax Notation One) support tool.The merits and shortcomings of two typical approaches in realization of an ASN.1 compiler are analysed.One of them translates an ASN.1 module into a tree-like internal representation and the other into the source codes of a programming language.The author combined the two approaches in an elegant one in the development of ASN.1 tool.As a result,it is not only suitable for protocol implementation,but also suitable for protocol testing.