Abstract: The influence of uniform magnetic fields on the dose distribution of carbon-ion pencil beams is analyzed, and the influencing effect is corrected to provide guidance for the clinical application of MRI-guided carbon ion radiotherapy. In this study, the Monte Carlo method was used to simulate the dose distribution of carbon-ion pencil beams with different energies under different magnetic fields. We found that the transverse uniform magnetic fields had little effect on the range shortening of the carbon-ion pencil beams, but had obvious impact on the lateral shift, especially at the Bragg peak of the carbon-ion beams. The lateral shift was related to the carbon-ion beam energy and magnetic field strength. According to the simulation results, we obtained an equation for calculating the lateral shift of carbon-ion beams under the magnetic fields, and proposed an angle correction method to reposition the Bragg peak of carbon-ion beams. These results will be used to MRI-guided carbon ion radiotherapy in the future.