Case based reasoning is a new way in AI. In view of the disadvantages existing in the present cased based,this paper elaborates them with new viewpoints, including case representation under complex circumstances,case retrieval,case adjustment and the integration between case based reasoning and other reasoning engine such as rule based reasoning and evidence based reasoning. An algorithm of case based complex reasoning under incomplete information is also put forward. The above study stimulates the application of case_based reasoning and provides it with theoretical suppot．Furthermore，based on the analysis of decision problem solution model，this paper discusses the solution strategy under complicated decision background using case—based reasoning and gives the framework of the system．