Metasurfaces, planar metamaterials consisting of a single layer or several layers of artificial structures, not only form the basis for fundamental physics research but also have considerable technological significance. Metasurfaces can locally modify the optical property within a subwavelength range, which can facilitate device miniaturization and system integration. Metasurfaces have shown extraordinary capabilities in the local manipulation of the light’s amplitude, phase, and polarization, leading to a plethora of novel applications such as generalized Snell’s Law of refraction and photonic spin Hall effect. In this paper, we will focus on the recent advancements in the fundamental research of geometric metasurfaces and their applications in ultrathin optical devices, including planar metalenses, helicity multiplexed holograms, functionality switchable devices, polarization beam splitters, vector beam generation, arbitrary polarization control and so on. The compactness, ease of fabrication and unusual functionalities of these devices render geometric optical metasurface devices very attractive for new applications such as encryption, imaging, anti-counterfeiting, optical communications, quantum science, and fundamental physics. This paper aims to bring readers some new insights, and to broaden the applications of geometric metasurfaces in more research fields of science and technology.