The Politeness Principle overcomes the shortcoming of the Cooperative Principle which can not explain why people tend to express their ideas in an indirect and implicit way. In diplomacy field, politeness is one of the basic norms governing diplomat’s behavior. Vagueness is always tactfully used by diplomats to achieve politeness, avoid conflict and so on, which conforms to the Politeness Principle. Among the maxims of Politeness Principle, the Tact maxim, Approbation maxim and Agreement maxim are frequently observed by diplomats.