The photon flux of the synchrotron radiation hard X-rays was measured in Beijing Synchrotron Radiation Facility (BSRF) using a free-air ionization chamber. Absolute air-kerma can be obtained with the free-air ionization chamber. The photon flux was calculated based on the relation between air-kerma and flux in the fundamental quantities and units for ionizing radiation. Because the photon flux changes over time during the measurement, an online monitoring system was applied, and the uncertainty of which is 0.03%. The air attenuation correction factor and ion recombination correction factor were measured at 15 keV synchrotron radiation X-rays. The measurement results show that the absolute air-kerma rate at 15 keV synchrotron radiation X-rays is 0.239 Gy/s, accordingly, the photon flux of the source is 5.28×109 photons/s, the relative uncertainty is 0.68%.