Since the advent of graphene, two-dimensional materials with various novel properties have received more and more attention in the fields of optoelectronic devices, spintronics and valley electronic devices. Among them, the excellent properties that appear in graphene with various molecular groups for asymmetric functionalization have led to the research of other Janus two-dimensional materials with asymmetric surface characteristics. As an important derivative of two-dimensional materials, Janus two-dimensional materials (especially Janus transition metal chalcogenides) have become a research hotspot in recent years. Both experiment and theory have confirmed that this kind of material has mirror asymmetry and novel characteristics, such as strong Rashba effect and out-of-plane piezoelectric polarization, and thus showing a great prospect for its applications in sensors, actuators, and other electromechanical devices. In this review we introduce the recent research progress of emerging Janus two-dimensional materials (including Janus graphene, various Janus two-dimensional materials and Janus two-dimensional van der Waals heterojunction), and summarize the unique electronic properties and potential applications of Janus two-dimensional materials. Finally, we draw some conclusions and depict a prospect of further exploration of Janus two-dimensional materials.