The frontier fortress poem of Tang Dynasty is one of the most profound ones in thoughts and the richest in imagination. Poem is the art of language while the rhetoric is one of the important means to improve the art of language. Metonymy in the frontier poems of Tang Dynasty is widely used as a kind of rhetorical devices. This article mainly studies the metonymy used in the frontier poems of Tang Dynasty from two aspects： the metonymy types and the rhetorical effect of metonymy.