The invention relates to a mounting device for mounting a remote radio unit to a base station antenna, including: a single guide rail (4) configured to be mounted to the base station antenna (1) and extending transversely in a mounting state of the guide rail on the base station antenna; a guide element (8) configured to be mounted to the remote radio unit (2) and movable along the guide rail in a mounted state of the guide element on the remote radio unit; and a fixing device such as a plurality of quick locking devices (10) configured to fix the remote radio unit on the base station antenna at a predetermined installation position of the remote radio unit relative to the base station antenna. The invention also relates to a base station antenna system, which includes a base station antenna, a remote radio unit and the above-mentioned mounting device. By use of such a mounting device, the remote radio unit may be easily mounted on the base station antenna, and the whole base station antenna system is compact in structure.