This paper, in the first place, introduces a labour force cost accounting model constructed by using SADT, then through the analysis of the given model expounds seven main styles of SADT and approaches several particularities about the activities decomposition. Finally, some useful conculusions about themethod are given out.SADT is a kind of method mainly for the structured Systems analysis and has come into being for ten years. But in our country, it still has not been systematically introduced as a whole. So, this situation might make this paper meaningful for the understanding and application of SADT.