Optical clocks have developed rapidly in the past 20 years, and their stability and systematic uncertainty are two orders of magnitude better than the current best microwave atomic clocks, and currently, there are 10 optical transitions which have been selected as the secondary representations of the definition of the second by the International Bureau of Metrology and participate in the generation of the international atomic time. This paper introduces the operational principle and evaluations of the performance of optical clocks, the latest research progress of ionic optical clocks and optical lattice clocks, elaborates the progress of the absolute frequency measurement of optical clocks, and summarizes the measurement results of the secondary representations of the definition of the second by optical frequency transitions.