Optical zoom plays an important role in realizing high-quality image magnification, especially in photography, telescopes, microscopes, etc. Compared to traditional bulky zoom lenses, the high versatility and flexibility of metalens design provide opportunities for modern electronic and photonic systems with demands for miniature and lightweight optical zoom. Here, we propose an ultra-thin, lightweight and compact bifocal zoom metalens, which consists of a conventional circular sub-aperture and a sparse annular sub-aperture with different focal lengths. The imaging resolutions of such single zoom metalens with 164 lp/mm and 117 lp/mm at magnifications of 1× and 2× have been numerically and experimentally demonstrated, respectively. Furthermore, clear zoom images of a dragonfly wing pattern have been also achieved using this zoom metalens, showing its distinctive aspect in biological imaging. Our results provide an approach for potential applications in integrated optical systems, miniaturized imaging devices, and wearable devices.