In order to carry out the measurement of heavy nuclei in accelerator mass spectrometry, characteristic X-rays of the incident projectile have been explored and used as a method for isobar discrimination. The projectile X-ray combined with AMS technique has been set up in China Institute of Atomic Energy. The measurement of 79Se was performed by resorting to the projectile X-ray AMS technique and the etection sensitivity of 79Se was improved more than 2 orders of magnitude. The detection limit was about 3.6×10-9 for 79Se/Se.