The parasitic effect inside metal oxide field effect transistor regarded as the basic structure of large scale digital circuits, has long been considered as an important factor affecting the disturbance, upset and latchup of integrated circuits in pulsed γ-ray radiation environment. To investigate the turn-on mechanism of vertical parasitic effect in NMOSFET induced by pulsed γ-ray radiation, the 40 nm, 90 nm and 180 nm NMOSFET device models are constructed by TCAD and the normal electrical characteristics are calibrated. The trend of vertical parasitic triode current gain, the turn-on conditions of vertical parasitic triode and their influence on working state of NMOSFET are obtained. The simulation results are shown below. 1) The disturbance of well potential inside NMOSFET induced by pulsed γ-ray radiation is the main reason for the turn-on of vertical parasitic triode. 2) When vertical parasitic triode is turn-on, the large secondary photocurrent will be generated inside NMOSFET which will affect the working state of the transistor. 3) The current gain of vertical parasitic triode in NMOSFET decreases with the technology node decreasing. The results provide a theoretical basis for studying the transient ionizing radiation effects of electronic devices.