Since 2005, active optical clock (AOC) has undergone nearly 20 years of development. The AOC utilizes an atomic ensemble as the gain medium, and its stimulated radiation can be used as the clock laser signal directly. Because the AOC works in the bad?cavity region, it has two significant advantages of cavity?pulling suppression and narrow linewidth, which can effectively overcome the cavity length thermal noise problem of the passive optical clock. Due to its superior performance, the AOC has received wide attention from international counterparts. According to the different implementation methods, this paper classifies AOCs into atomic beam type, laser cooling and optical?lattice?trap type, atomic beam and optical lattice "hybrid" type, Faraday atomic filter type, ion?trap type, and thermal atomic cell type. For different types of AOCs, this paper presents the experimental and theoretical research progress in detail and analyzes their advantages and disadvantages. Finally, the application of AOCs in the field of precision measurement is analyzed, and the future development direction of AOCs is prospected, so as to provide reference for promoting the wide application of AOCs.