In order to guide and inspire the designer’s thinking efficiently, this paper puts forward the concept of design decision context, which is an abstract expression of design rationale knowledge models by ignoring design trial-and-error processes and iterations. Design decision context is the core causality development path of design thinking. Based on the proposed fine-grained design rationale knowledge model, this paper proposes a design decision context mining method. Firstly, a hierarchical design decision model is constructed based on the quotient space theory, which can support the designer to analyze the design problem from different levels of granularity. Secondly, semantic reduction of the model is realized through the improved manifolds ranking method. Finally, the core design thinking process, i.e., design decision context, is obtained by eliminating the less relevant knowledge fragments. The proposed mining method is verified by a developing a prototype system.