Irony, as a universal language phenomenon, draws great attention from various fields of philosophy, psychology, cognitive science, achieving fruitful results in both theoretical and empirical studies. However, an agreement hasn't reached yet on the issue of the production, interpretation and even definition of irony. Hence, based on previous studies, the paper proposes a new idea, claiming that irony is in essence implied negation, expressing the meaning of ~P by P without negation markers. With this definition, conceptions with similar usage can be distinguished, like understatements, litotes, metaphors and sarcasms. Irony can be classified into propositional negation irony and ideational reversal irony and its pragmatic functions can be divided into positive kinds, including humour, compliment and intimacy, and negative kinds, including criticism and sarcasm. Irony plays an important role in our daily communication, reflecting the vividness and interesting side of language. The study of irony facilitates further investigation of language and meaning and interpersonal communication.