Recently, there are limited specific researches on the pragmatic strategies of teachers' criticism from the angle of the Politeness Principle, among which most of them argue that teachers should adopt appropriate politeness strategies when criticizing students. However, only a few researchers point out that Leech's Politeness Principle is "other" oriented and neglects "self," which is too absolute, ideal, and unsuitable in teacher and student relations. In this paper, the author illustrates the true nature of politeness in teacher-student relations is that teachers should pay attention to self, show politeness to the students, and consider the third party (parents). Finally, the writer illustrates some politeness strategies, including criticizing with a sense of humour, criticizing in vague language, adopting empathy and showing politeness to a third party to show appropriate politeness to the students to make the criticism effective. The purpose is to make some middle school teachers recognize how to criticize appropriately.