Abstract: With the technology of parametric design，the data of design cases are extracted from graphics．A new method of case representation，CASE＝DATA ＋STRUCTURE，and a kind of synthesis method of case-based reasoning（CBR） based on knowledge are proposed．The conception of decision tree for cases is described．The problem of case extraction is converted into the multiple index of database．The design of hydraulic cylinders is taken as an example for how to realize these methods．