The ion energy distribution and average energy of Ar/O-2 mixed gas discharge have been studied using quadrupole mass spectrometer. The results show that the potential increases in the sheath, the ions obtain higher energy through the sheath, high energy peak moves to high energy area and the energy width increases as low-frequency power increases, which is consistent with the theoretical model calculation results. When pressure rises, collisions of Ar+ ions are more frequent in the process of crossing the sheath, the bimodal structure becomes vaguely, and under the same condition the number of low energy ions increases gradually.