To investigate the turn-on mechanism of vertical parasitic effect in NMOSFET induced by pulsed γ-ray radiation. 40nm, 90nm and 180nm NMOSFET device models were constructed by TCAD and the normal electrical characteristics were calibrated. The trend of parasitic effect, the turn-on condition of parasitic bipolar transisor and their influence on transient ionizing radiation effect of NMOSFET were obtained. The simulation results show that 1) The disturbance of well potential inside NMOSFET induced by pulsed γ-ray radiation is the main reason for turn-on of vertical parasitic bipolar transistor. 2) When vertical parasitic bipolar transistor is turn-on, large secondary photocurrent will be generated inside NMOSFET which will affect the operation state of the transistor. 3) The magnification of vertical parasitic bipolar transistor in NMOSFET decreases with the reduction of technology node.