An extensional reasoning model for complex product scheme design based on extension ontology correla-tion related network was put forward to solve the extension reasoning issue of relevant constraints in complex pro-duct scheme design． Firstly，an extensional ontology model for complex product scheme design was constructed，and the relativity of extensional ontology was analyzed． Secondly，the direction of product scheme design was obtained through domain knowledge，and the extension correlation function of extension ontology features was pro-posed． The extensional ontology correlation network was constructed based on the information model． Then，an extension reasoning algorithm for scheme design based on extensional ontology related network was presented． The initial product design scheme meeting design requirements was obtained through multi-level grey relational cluste-ring analysis of design direction and relational degree calculation analysis based on extensional ontology related net-works． Finally，the models and the algorithms were illustrated and validated by a design case of hydro turbine． The results show that the models and the algorithms are effective and feasible．