Abstract: In the paper, the elements and their interrelations in the process model of mechanical product conceptual design, including function, behavior, structure and environment constrained are described rationally with mathematical method. The whole process of conceptual design is formalized, including function information extraction in terms of product specification, establishment of criterion for function decomposition, establishment of behavior relation trees and conceptual solutions for implementing product function. Finally in the study, the method is applied to the conceptual design of a filling material system to verify its validity.