It is distinctive that English advertising slogans feature in unique and discourse characteristics. On the ground of modality system in interpersonal meta-function, about 120 pieces of English slogans with modality are randomly selected as samples from 500 famous advertisements. These slogans are systematically analyzed from the perspective of modality type, modal value and modal responsibility, aiming at interpreting the functional characteristics of English slogans. The result is that English slogans usually employ modalization, with median or low modality value, accompanied by expressive subjective orientation. It is anticipated that this article can render guides for advertisers to compose advertising slogans effectively and enhance the ability of advertising speech communication.