"Taishang zuozhe zhuxituan" (lexically: platform sits presidium)as a special construction in Chinese has been discussed for over 70 years and no consensus has been reached up till now. The theory of grammatical metaphor in systemic functional linguistics is used here to explain the characteristics and the mechanism of the construction in a systematic way.According to this theory, the construction is a typical ideational grammatical metaphor formed out of downward rankshifting of the clause complex "nashi zhuxitai, zhuxituan zuozai taishang"(That is the platform; The presidium sits on the platform) which is composed of a relational process and an existential process. Similar rank-shifted constructions may be fossilized and become productive forms in language use when they manifest as special clauses. Taishang in the clause can thus be the Subject (interpersonal function), the Circumstance (ideational function) and the Theme (textual function) simultaneously.