Mammography x-rays standard is required to improve the quality of mammary machine and reduce the dose for women and staff. The molybdenum target x-rays machine is used as the radiation source and free-air cylindrical chamber is designed for detecting photon. Four standard radiation qualities are established with international standard documents as references. The Correction factors including air attenuation, scattered radiation, fluorescence, wall transmission, electron loss are studied in the four radiation quality. The uncertainty in absolute measurement of air-kerma includes air volume measurement of ionization chambers, current measurement, measurement of temperature and pressure, measurement of correction factors, physical constants and orientation of the reference distance. The relative combined standard uncertainty is 0.24%.