This essay is based on metaethics. First, it aims at investigating the central viewpoint of metaethics by pointing out the differences between normative ethics and metaethics, as the latter distinguishes moral language and contents of moral language from traditional ethics, and disclaims the possibility that 'is proposition' can prove the contents judgment. Secondly, this essay aims at introducing the basic claims of analysis on moral concepts and judgments of three schools of metaethics, i.e. intuitionism, emotivism and prescriptivism. Finally, we point out five common faults of the three schools of metaethics and these cause their downfallen, which is the conclusion of this essay.