This application provides an antenna apparatus and a base station antenna system. The antenna apparatus includes a reflection panel, a radiating element, and a feed network. A first aperture is provided on the reflection panel. The radiating element includes a radiator and a feed stub, and the radiator is coupled to the feed stub. The radiator is disposed on one side of the reflection panel and is spaced apart from the reflection panel. At least a portion of the feed stub passes through the first aperture. The feed network is disposed on a side, facing away from the radiating element, of the reflection panel and is spaced apart from the reflection panel. The feed network is coupled to the feed stub. This simplifies a structure of the antenna apparatus, reduces a quantity of components, reduces a weight, and facilitates overall assembly of the antenna apparatus.