Abstract: With the calibrated silicon photodiode as standard detector, the sensitivity of X-ray diodes (XRDs) with different photocathodes (Al and Au) has been absolutely measured on the Beijing Synchrontron Radiation Facility. The 2nd-order harmonic in the light source will cause the derived sensitivity to decrease. In order to eliminate this effect, a transmission grating was used to measure the share of the 2nd-order harmonic, and the result was utilized to correct the derived sensitivity of XRDs. A simple model for secondary electron emission of photocathode was presented, and the conversion efficiency of secondary electron was analyzed. The sensitivity in the energy range without experimental data was then calculated through the model. The uncertainty analysis was also carried out, and the resu