Abstract: Radiotherapy level ionization chamber dosimeter is the main measuring instrument for radiotherapy in hospitals, and it is also the national compulsory verification measuring instrument. The accuracy of its dose measurement is the key carrying out precision radiotherapy. To ensure the accuracy and unity of the quantity value, from 2019 to 2020, National Institute of metrology, China, as the leading laboratory, organized the national comparison of the 60 ~ 250 kV X-ray air kerma. A total of 11 laboratories participated in the comparison, and all reference laboratories established 60 kV, 100 kV, 135 kV, 180 kV, and 250 kV reference radiation. The calibration factor and measurement uncertainty under different radiation were obtained by calibrating two transfer standard ionization chambers in the radiation field using the petal comparison method. The comparison results show that the normalized deviation of each reference laboratory is less than 1, and the comparison results are satisfactory.