Abstract: Ion beam radiotherapy is increasingly employed in cancer treatment. When using conventional X-ray computed tomography(CT) for ion radiotherapy treatment planning, a significant issue exists: the relative stopping power(RSP) exhibits substantial errors. An ideal approach is to directly utilize high-energy ion beams for patient imaging, generating the medical images required for ion radiotherapy treatment planning, thereby avoiding RSP conversion and reducing RSP errors. This study employed the Monte Carlo simulation toolkit Geant4/Gate to establish an ion CT simulation platform. Idealized and realistic ion CT imaging system detectors were designed. Image reconstruction was achieved using the Maximum Likelihood method and the ASD-POCS algorithm. The research investigated the impact of both idealized and realistic setups, various energies, and different ion species on the RSP reconstruction errors for four material inserts(sulfur, phosphorus, carbon, and calcium) within a phantom. The results indicate that for 330 MeV protons, the relative RSP error was less than 1.547% under both idealized and realistic setups. The RSP error in the idealized setup was significantly lower than in the realistic setup; in the realistic setup, the RSP reconstruction errors for various materials were nearly triple those observed in the idealized setup. The relative RSP error for protons decreased with increasing energy. The maximum error occurred at 230 MeV, with relative RSP errors for sulfur, phosphorus, carbon, and calcium being 2.855%, 2.468%, 1.653% and 2.553%, respectively. The minimum error was achieved at 330 MeV, with relative RSP errors of 0.181%, 0.351%, 0.250% and 0.245% for sulfur, phosphorus, carbon, and calcium, respectively. At an energy of 330 MeV/u, carbon ions exhibited relative RSP errors of 0.060%, 0.281%, 0.150% and 0.082% for sulfur, phosphorus, carbon, and calcium, respectively. All errors were within 0.281%, and the relative RSP errors were lower than those for 330 MeV protons. Therefore, compared to proton CT, carbon ion CT is expected to provide more accurate RSP data for ion radiotherapy treatment planning.