Abstract: The neutron irradiation effects and mechanisms of enhancement-mode Cascode-structured GaN HEMT devices were investigated using neutrons with a normalized energy of 1 MeV and a flux of 1×10¹⁴ n/cm². The electrical characteristics of the device before and after the irradiation were tested. It is found that after neutron irradiation, the threshold voltage of the device undergoes a significant negative drift, and the transconductance peak value decreases. Subsequently, Geant4 energy deposition and TCAD irradiation damage simulations were carried out for the enhancement-mode Si MOSFETs and the depletion-mode GaN HEMTs cascaded in the device, respectively. The results show that the energy loss and electrical performance degradation of the enhancement-mode Si MOSFET device are more serious. The reason is that the displacement damage to the device caused by neutron irradiation, and the ionization damage to the device caused by the secondary re-core lead to an increase in the electric field strength at the Si/SiO₂ interface and a decrease in the internal carrier concentration. Therefore, the threshold voltage drifts negatively, and the saturated drain current drops. The proposed research may provide theoretical reference for the application of enhancement-mode Cascode-structured GaN HEMT devices in irradiation environment.