The equipment and devices which are long-time running in space are affected by space radiation effects and hot carrier injection effects at the same time which would reduce their optional times. Normally, the single mechanism test simulation method is used on the ground simulation test but the multi-mechanism effect affects the space equipments and devices, including total irradiation dose effect, hot carrier injection effect, etc. The total dose dependence of hot carrier injection (HCI) effect in the 0.35 m n-channel metal oxide. semiconductor (NMOS) device is studied in this paper. Three samples are tested under different conditions (sample 1# with total irradiation dose (TID) and HCI test, sample 2# with TID, annealing and HCI test, sample 3# only with HCI test). The results show that threshold voltage of NMOS device with 5000 s HCI test after 100 krad (Si) total dose radiation has been negatively shifted then positively during total dose irradiation test and HCI test, and the threshold is higher than that of the device without radiation test. But the threshold voltage shift of NMOS device with 5000 s HCI test and 200 h annealing test after TID test is higher than that of the devices without radiation test and lower than that of the devices without annealing test. That is, the parameters of NMOS device vary faster with the combined effects (including the total dose irradiation effect and HCI effect) than with single mechanism effect. It is indicated that the hot electrons are trapped by the oxide trap charges induced by irradiation effect and then become a recombination centre. And then the oxide trap charges induced by irradiation effect reduce and become negative electronic. The interface trap charges induced by irradiation effect are reduced and then increased it is because the electrons of hole-electron pairs in the Si-SiO2 interface are recombined by oxide traps in the oxide during the forepart of HCI test but then the electrons are trapped by interface traps in the Si-SiO2 interface because the electrons from source area are injected to interface during the HCI test. So the threshold voltage is positively shifted due to the negative oxide trap charges and interface trap charges. The association effect is attributed to the reduction of oxide traps induced by recombination with hot electrons and the increase of the interface traps induced by irradiation trapped hot electrons.