Abstract: The space charge effect is one of the key factors affecting ion beam transport and ion beam quality, especially for low-energy high-intensity ion beams. It can be partially compensated by the secondary electrons, which are produced from the ionization of residual gas molecules in the beam pipe and trapped by the space charge potential of ion beams. To study high intensity beam transport in low energy section, it is essential to have an accurate value of space charge compensation degree(SCCD) of the beam, particularly for mixed ion beams. Using a three-grid energy analyzer and a beam profile monitor based on an 128-channel picoammeter system, we measured the secondary ion energy distribution and the beam current distribution of mixed oxygen ion beams of different beam intensities and beam distributions, respectively. Thus the SCCD of the beams can be calculated. The results show that for mixed oxygen ion beams of different intensities, the SCCD are basically around 70% under the vacuum of 1.0×10⁻⁵ Pa; The space charge potential of the beams are affected greatly by the beam current distributions, which also consequently have influence on the SCCD of the ion beams.