It is difficult to obtain the practical knowledge of design and convert it into rules for constructing a CAAD expert system. In order to support the design thinking processes at the initial stage, a CAAD expert system framework based on the Case-Based Reasoning(CBR) is proposed in this paper. The proposed system has following characteristics: 1)The CBR method which is suitable to the idea design is used in our system. 2)Not only the thinking informations, which include the design know-how such as the design drawings,intention and approachs, but also the used architecture code are abstracted as the contents of case. 3)CBR and Rule-Based Reasoning(RBR) are integrated into the inference mechanism to improve the inference ability of the system.