Abstract:  Recently, with the booming development of the mobile Internet and the rapid popularization of smart terminals, the requirements for seamless network coverage and high-speed data transmission are increasing. Ultra-dense heterogeneous cellular wireless networks improve wireless resource utilization and spectrum efficiency through reducing the radius of cells and increasing the number of low-power nodes for grouping ultra-dense networks, which attracts widespread attention from international researchers. However, the increasingly dense network deployment in ultra-dense heterogeneous cellular wireless networks also brings a variety of serious problems, including system interference, the secure transmission of wireless information, and the energy supply problem of network operations. To this end, the ultra-dense heterogeneous cellular wireless network architecture is firstly introduced. Then some key technologies and current challenges in ultra-dense heterogeneous cellular wireless networks such as interference management, physical-layer security, and energy efficiency are analyzed. Finally, the future development of ultra-dense heterogeneous cellular wireless networks is discussed.