Abstract: The relative biological effectiveness (RBE) of carbon ion beams was further obtained by calculating the DNA damage of carbon ion beams on human salivary gland (HSG) cells through the simulation of a water phantomand microscopic cells.Using the Monte Carlo software GATE/Geant4, the incidence of a carbon ion beam with a single energy of 400 MeV/u in a water phantom was simulated to obtain the dose and energy spectrum of ¹²C as well as fragment elements at different depths, and based on the energy spectrum, the Monte Carlo damage simulation (MCDS) software was used to calculate the DNA damage caused to human salivary gland (HSG) cells on the basis of the energy spectrum, so as to obtain the DNA double-strand break (DSB) yield of carbon ion beam, ¹²C and its fragments, and thus the RBE values.The results confirmed that the carbon ion beam had a high RBE, suggesting an advantage of using the carbon ion beams to treat tumors.In addition, by changing the parameters, the simulation method can be extended for studies, providing an important reference value for a deeper understanding of carbon ion beam radiation therapy.