Abstract: In carbon ion radiotherapy, secondary particles produced through the interaction of carbon ion beam with the treatment head devices and the patient's body can reach many areas in the patient's body. Among the secondary particles, the yield of neutrons and \gamma -rays is largest. Without affecting the beam delivery function, reducing secondary neutrons and \gamma -rays produced in carbon ion radiotherapy is very important to decrease the normal tissue complications and secondary cancer risk after radiotherapy. In the present work, the Monte Carlo method was used to calculate the dose and spatial distributions of secondary neutrons and \gamma -rays deposited in water when multi-leaf collimators (MLC) made from different material leaves were adopted to form a typical 10 cm×10 cm square irradiation field for 400 MeV/u carbon ion beam. The simulation results showed that the secondary neutrons produced were mainly distributed at the incident end in the water phantom when the carbon ion beam passed through the MLC, while the secondary \gamma -rays were uniformly distributed in the entire water phantom and more dose of the secondary \gamma -rays appeared in the plateau when the irradiation field with spread-out Bragg peak (SOBP) travelled in the water phantom. The selection of MLC leaves should be determined according to the actual conditions of MLC leaves thickness and secondary particle equivalent dose requirements. The simulation study presented in this paper provides scientific evidence for the material selection of MLC leaves and other components in carbon ion radiotherapy using passive beam delivery system.