The Heavy Ion Cancer Therapy which is known as the most advanced cancer therapy means had been carried out at National Institute of Radioactivity Sciences (NIRS Japan) in 1900s[1]. In 2006, IMP carried out some heavy ion cancer therapy experiments. During the treatment, particle fragments from heavy ion reaction will influence the treatment dose and healthy tissue of patient. Neutron is the most abundant secondary particles in heavy ion reaction, may influence large area of patient body due to its strong penetrating power, thus it is important to know the neutron field in heavy ion therapy to evaluate the neutron impact and assess the patient safety. Neutron spectra and dose equivalent distributions from 430 MeV/u C ions stopping in thick solid water target were calculated by Fluka Monte Carlo code[2]. The results were shown in Figs.1 and 2 respectively. For comparison with experimental result, the neutron spectra of 400 MeV/u C ions stopping in copper target were calculated. The Heavy Ion Cancer Therapy which is known as the most advanced cancer therapy means was already carried out at National Institute of Radioactivity Sciences (NIRS Japan) in 1900s[1]. In 2006, IMP carried out some heavy ion cancer therapy experiments. During the Treatment, particle fragments from heavy ion reaction will influence the treatment dose and healthy tissue of patient. Neutron is the most abundant secondary particles in heavy ion reaction, may influence large area of ​​patient body due to its strong penetrating power, thus it is important to The understanding of neutron field in heavy ion therapy to evaluate the neutron impact and assess the patient safety. Neutron spectra and dose equivalent distributions from 430 MeV/u C ions stopping in thick solid water targets were calculated by Fluka Monte Carlo code[2]. The Results were shown in Figs. 1 and 2 respectively. For comparison with experimental result, the neutron spectra of 400 MeV/u C ions stopping in copper targets were calculated.