With the development of Fifth-generation (5G) mobile communication, mobile networks are dedicated to accommodating more subscribers and diversified service scenarios. 5G is then expected to significantly enhance the quality of experience on services, and pave way for the interconnection of everything. Starting from the development trends of radio accoss network (RAN), this paper investigates its evolution progress and key technologies, including wireless access, optical bearing, interfaces for fronthaul and midhaul, control and management, as well as network slicing. Finally, the urgent challenges are described from the perspective of RAN evolution towards 5G.