The present disclosure provides a backlight device and a display apparatus. The backlight device includes: a light guide plate having a light exit face and a light entrance face, and a plurality of light extraction structures disposed on the light exit face. A light beam is totally reflected in the light guide plate, and can enter the light extraction structure through a surface of the light extraction structure that is in contact with the light exit surface of the light guide plate. The light extraction structure is configured to converge its output light beam when the backlight device is used in a dark display state, and diverge its output light beam when the backlight device is used in the grayscale display state.