Abstract A new full-orbit Monte Carlo code (SOFT) has been developed and used to investigate how resonant magnetic perturbation (RMP) affects the loss of neutral beam injection ions in EAST. Benefiting from the calculation of real orbits in cylindrical coordinates, the simulation can take into account the first wall and provide a more realistic evaluation of the losses than previous results. The two co-current beams in EAST are chosen for the study. Depending on the spectrum used, losses can be significantly enhanced by low-n (n ≤ 2) RMPs. From the quantity perspective, the prompt loss and the resonant loss are the two loss channels of concern. The former is mainly related to the ion source, while the latter is closely related to RMP and quantitatively more dominant. It is found that both linear and non-linear resonances play an important role, which is consistent with previous results. The inclusion of plasma response, to a large extent, heals the magnetic topology, but does not necessarily lead to a better confinement of fast ions. Detailed analyses reveal the significant loss of passing ions in the presence of the response, which highlights the importance of the non-resonant components. A better understanding of the loss channels and their relation to the RMP spectrum helps to avoid the detrimental effects and to provide support for goals like phase-space engineering.