This paper mainly studies the design on double level case-based reasoning mechanism. Firstly, it discusses the rationale and method of case-based reasoning (CBR) which is treated as outer case-based reasoning. Secondly, it proposes the mathematical model, modification principle, construction of modification model, matching algorithm, learning and maintenance of case-based modifying. This is the inner case-based reasoning. This design on double level case-based reasoning mechanism deepens the research on CBR, expands limitation and establishes certain groundwork for further development.