Abstract: MSAT is an analysis and synthesis software particulariy for the designing of mechanical construction units. Its performance and characteristica are specifically introduced in this paper. On the basis of "the Design Sensitivity AnalyAnalysis", a non-linear quadratic programming is adopted to the "MSAT", So as to make full use of the technology of Optimization and the finite Element Methods.Also a series of measures are taken to Speed up the calculation. It is well approved that the "MSAT" is an effective tool in practice. It works efficiently and accurately for mechanical construction unit design.