Abstract: On the basis of 250-600 kV X-ray machine, the narrow spectrum series X-ray radiation qualities in the high-energy section were established, and the graphite cavity ionization chamber was developed. The 250-600 kV X-ray air-kerma was measured by Monte Carlo simulation and experiment. Under the condition of 300 kV radiation quality, the relative standard uncertainties are 0.61% and 0.45% by using free air ionization chamber and graphite cavity ionization chamber. The relative deviation between the two methods is 0.09%. The absolute measurement of air-kerma in ¹³⁷Cs and ⁶⁰Co γ-ray reference radiation field was completed by using the graphite cavity ionization chamber, and the relative deviations between the results and the reference value are 0.27% and 0.39%, which are equivalent within the range of uncertainty. The feasibility of measuring air-kerma of 250-600 kV X-ray by using graphite cavity ionization chamber was verified.