Abstract: The relationship between the threshold voltage and the back-gate voltage of fully depleted DSOI NMOS is no longer satisfied with a single linear relationship after high total dose irradiation or high backgate voltage. The coupling mechanism between the threshold voltage and the backgate voltage will change after the back surface of the fully depleted DSOI device changes from the depletion region to the reversion region. Previous model is not enough to describe this phenomenon. In order to solve this problem, a new threshold voltage model was proposed considering the influence of backgate current. The coupling relationship between the threshold voltage and backgate voltage of the devices suffered from a high backgate voltage or high total dose irradiation can be well fitted.