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Type system provides a precise description of a programming lanrnguage. This is a prerequisite for the implementation and use of language. It also rnconducts mechanical and transparent type-checking on programs to prevent the ocrncurrence of execution error during the running of programs. So, it can be said that,rnon the one hand, type system works as a formal tool to do mathematical analysis rnof language; on the other hand, it is a formal method for rigorously and precisely rndesigning and implementing language. In this paper, some basic concepts of type rnsystem are discussed first. And then, the implementation of a graph-rewriting-based rnfunctional language － SClean’s type system is given in details. It is hoped that the rnproposed method of using and implementing type system is of practical usefulness.