Based on the hybrid reasoning method of rule-based reasoning (RBR) and case-based reasoning (CBR), this paper studies the intelligent design method of aircraft structural parts. Firstly, combined with fuzzy theory and rulebased reasoning technology, processing fuzzy parameters for aircraft structure parts design, and the intelligent selection system of aircraft structural parts is designed to realize the type selection of aircraft structural parts. Secondly, the K-nearest eighbor (KNN) and grey relation analysis (GRA) are used to deal with the numeric parameters of aircraft structural parts  design, and it implements cases retrieval and evaluation. Then the best similar cases are used for variant design to obtain the target case. Finally, the feasibility of the hybrid reasoning intelligent design system is verified by taking the section steel as an example.