Abstract: Modality is an important aspect of interpersonal metafunction in Systemic Functional Linguistics (SFL). It is closely related to the Speaker's sex and age. This article aims to investigate their influence on modality type of COULD in Harry Potter and its Chinese translation, within the framework of modality system in SFL and by means of WordSmith Tools and SPSS. Results show that COULD does not express "usuality"in ST, but in TT it can be translated into "usuality". Translators tend to use modal operators and modal adverbs to render adult speaker's COULD as"probability".