Abstract: With the development of Magnetic Resonance Imaging(MRI) technology, the role and importance of image guided radiation therapy in radiation oncology are increasing rapidly. To develop the technique of MRI-guided heavy ion radiotherapy, the influence of uniform magnetic fields on the dose-averaged LET and nanodosimetric quantities of carbon-ion beams is analyzed. In this work, the GEANT4 kernel-based GATE Monte Carlo simulation platform was used to calculate the dose-averaged LET and nanodosimetric quantity distributions of carbon-ion beams with different energies under different magnetic fields. Compared to the cases without magnetic fields, it was found that the longitudinal uniform magnetic fields had little effect on the dose-averaged LET and nanodosimetric quantities of the carbon-ion beams. The influence of the lateral uniform magnetic fields on the dose-averaged LET and nanodosimetric quantities of the carbon ion beams mainly occurred in the Bragg peak regions, which was mainly caused by the lateral deflection of the carbon ion beams under the Lorentz forces in the magnetic fields, especially the lateral shift of the Bragg peak position of the carbon ion beams. These results provide a solid foundation for further study on how magnetic field affects the therapeutic performance of carbon ion beams.