Abstract: In carbon-ion radiotherapy, a variety of secondary particles are produced when carbon-ion beams pass through beam-line components during the dose delivery process and form an external radiation field. The predominant part of external radiation is from neutrons. As high LET radiation, low-dose neutron exposures have high relative biological effectiveness. Reducing the secondary neutrons can help to curb the risks of normal tissue complications and secondary cancers. The fluence and dose equivalent spectrums of exogenous neutrons induced by the passive beam delivery system of carbon ion therapy at different locations around the isocenter were simulated using the Monte Carlo method to evaluate the dose equivalent contribution of neutron radiation; The influence of the different collimating aperture sizes on the secondary neutron dose equivalent was also investigated. The simulation results show that high-energy neutrons had a certain contribution to the total neutron dose equivalent of the simulated locations, which account for up to 26%. Under real treatment conditions, as the collimating aperture size increased from closed to 15 cm×15 cm, the neutron dose equivalent decreased by approximately 85% at the isocenter and laterally distal locations, and about 70% at the distal locations in the beam direction. These results can be used to guide the study of secondary neutrons under different treatment conditions, and provide a basis for assessing non-therapeutic radiation received by patients under treatment conditions.