Abstract: The present paper, based on the theory of grounded cognition and after redefining modal symbols, explores the relationship between propositional and modal symbols.It is found that the operation of discourse basically means the mutual transformation between modal symbols and propositional symbols; the process of discourse production means transforming modal symbols into propositional symbols, with the latter as the embodiment of the former, whereas the process of discourse construal means transforming propositional symbols into modal symbols.Therefore, it can be concluded that modal symbols, penetrating the whole process of discourse operation, are the base of propositional symbols.This study can provide a new perspective for the study of discourse operation.