Modality is an everlasting subject of study, which has long been one of the research subjects for logicians, philosophers and linguists. Since a language is a dynamic system, there must exist some relationship between one subsystem of the language and other systems. The relationship between Modality and Mood has been discussed, mainly analyzing the modal meanings of imperative clause, conditional clause, purposive clause, concessive clause and subjunctive mood and it has been suggested that mood can be served as the grammatical expressions of modality.