Abstract: Assembly semantics is the abstract and generalization of assembly constraints among parts,thus using semantics can prominently enhance the straight forwardness and efficiency of assembly design.This paper puts forward an overall framework and essential functions of semantics guided assembly design system,classifies the assembly semantics into general ones and particular ones,resulting in a paradigm of hierarchical representation of assembly semantics.By direct mapping from assembly semantics to geometrical constraints,the algebraic expression of geometrical constraints is established and the degrees of freedom of parts motion are solved.In such a way,the assembly semantics can be used to drive automatically the execution of assembly process,that helps to realize the assembly design with semantic instructions instead of geometric ones.