Deep ultraviolet (DUV) light has a large potential application in sterilization, biochemical detection, UV curving, UV communication, and so on. AlGaN based DUV light emitting diodes (LEDs) have attracted tremendous attention and research because of their unique advantages such as nontoxicity, small size, low power consumption, long service life and wavelength tunability. After nearly 20 years research and development, the emission efficiency and device lifetime of AlGaN DUV LEDs have been promoted significantly, and some products have been commercialized. However, compared with the GaN based blue LEDs, the efficiency of current AlGaN DUV LEDs is still very low, which means the promotion room is very large for the research community. This paper first introduces the research status of the stateofart AlGaN DUV LEDs, and analyses the reasons for the low emission efficiency. Then, the recent progresses of the AlGaN DUV LEDs, from the internal quantum efficiency (IQE), light extraction efficiency (LEE), and wallplug efficiency (WPE), respectively, have been systematically reviewed. The various solutions that improve the efficiencies have been summarized. Finally, the future development directions and possible solutions for the efficiency have been provided.