This paper reviews the measurements of absolute frequency of the transition of calcium ion optical frequency standard, made by the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, and describes the history of the absolute frequency measurement results accepted by the Consultative Committee for Time and Frequency (CCTF) of the International Committee for Weights and Measures (CIPM). During the years between 2011 and 2020, based on the laboratory optical frequency standard and the transportable optical clock, by using the femtosecond optical frequency comb both referenced to the SI second through the satellite links, and directly traced to a cesium fountain clock made by the National Institute of Metrology (NIM), the absolute frequency of the clock transition for the calcium ion optical frequency standard was measured for a few times, and the measurement uncertainty was gradually improved from the order of 10^-15 to the order of 10^-16. Four measurement results were adopted by the CCTF to participate in the calculation of the internationally recommended frequency of the calcium ion optical clock transition, and the recommended value was updated for four times in 2012, 2015, 2017 and 2021, respectively. The calcium ion optical clock transition was recommended in 2021 as a new "secondary representation of the SI second".