Abstract: The theory of " ancient constitution” makes an important theme in constitutional ideas in near-modern Britain, while its actual status, function and influence have been much misread in both Chinese and western academy. A re-discussion shows that the theory of " ancient constitution” did not occupy an obvious place in the British constitutional discourse in early 17th century, neither a major theoretical source in constitutional debates. In its practice, the " ancient constitution” did not realize its original theoretical aim to restrict the power of the King, and, oppositely, it is used by lawyers to testify absolutism, resulting in the bankruptcy of the theory of " ancient constitution” in its common law form. Although the discourse of " ancient constitution” was still popular superficially in late 17th century, it became, in fact, a " disguise” of the discourse of the newly thriving constitution of the Parliament sovereign, and was declining with its own theoretical crisis. The study of the theory of " ancient constitution” must reiterate its essence of the discourse of ideology, and resume its real historical status and significance by dispelling all unrealistic misreading and myths.