Abstract In carbon-ion radiotherapy (CIRT), secondary neutrons are produced by nuclear interactions in the beamline devices or patient. Herein, the characteristics of secondary neutrons in CIRT with energy scanning (ES) were evaluated. Neutron ambient dose equivalents (H*(10)) were measured using WENDI-II. The neutron energy spectrum was calculated using the Monte Carlo simulation. Measurement and calculation were performed under realistic case scenarios using maximum beam energies (Emax) of 290, 350 and 400 MeV u −1. Moreover, H*(10) in ES was compared with H*(10) in range-shifter scanning (RS) and hybrid scanning (HS). H*(10) in Emax = 290 MeV u−1 was 65% less than that in Emax = 400 MeV u−1. At Emax = 350 MeV u−1, H*(10) in ES at θ = 120 was 42% of that at θ = 60. The neutron dose in ES CIRT decreased to approximately 60 and 70% of that in RS and HS CIRT, respectively, at 50-cm distance from the beam axis.