Abstract: To explore the similarities in the semantic cognitive mechanisms of English and Chinese idioms, this paper elaborates on the dynamic cognitive process of English and Chinese idioms based on the Mental Spaces and Mapping Theory of Cognitive Linguistics. It uncovers and discusses the shared traits and underlying reasons in the semantic construction of these idioms. Through the observation, analysis, and discussion of the corpus consisting of 1710 pairs of English and Chinese idioms, the paper reveals a key commonality: the meanings of elements in the source domain and input space tend to generalize. This generalization occurs across metaphorical, metonymic, and conceptual integration mappings, where the representational meanings of elements in the source domain and input space expand, exhibiting a clear trend toward broader conceptualization.