Poetry is a form of literary art expressed in words which only appear on the surface. Iconicity and metaphor, functioning under a common cognitive mechanism from a cognitive perspective, inevitably exist in poetry and affect its construction of words, images, concepts and so on. Based on Charles Sanders Peirce’s Theory of Iconicity and the Conceptual Metaphor Theory proposed by George Lakoff and Mark Johnson, this paper makes a detailed analysis and discussion of the operating mechanism and function of iconicity and metaphor in constructing Li Shangyin’s poem A Note on a Rainy Night to a Friend in the North from a cognitive perspective, supplemented by relevant examples.