This article takes "out of sight, out of mind" as an example to discuss the semantic structure and translation of English prepositional idioms construction from the perspective of cognitive construction grammar. There are roughly five elements in the semantic structure of prepositional idioms: the image schema, that is, the orientation relationship of the preposition itself, the logical subject of the prepositional idioms, the prepositional object, the overall construction and the context, etc. There are construction variants of prepositional idiom constructions, which reflect the inheritance and coercion relationships among constructions and the experiential nature of language. Idiom translation should deal with the relationship between the general and individual. The general semantics of "out of sight, out of mind" is "forgetting", and "not having in sight, and not bothering" and other semantics belong to individual semantic categories.