Based on 3D-TCAD simulations, the sensitivity region of SET(single-event transient) effect in 22 nm FDSOI (fully depleted silicon-on-insulator) devices were investigated. A comparison was made between the methods of using a single transistor and using an inverter to study the sensitivity region of device SET, in order to analyze the influence of heavy ion strike position on the SET sensitivity of 22 nm FDSOI devices in actual circuits, and to explain it from the perspective of charge collection mechanism. In depth analysis reveals that the parasitic bipolar amplification effect is sensitive to the position of heavy particle strike, which is the reason for the different sensitivity of SET in different regions of the device. The increased sensitivity of the drain caused by a constant voltage source connected to the drain of a single transistor is the reason why the SET sensitive area of the device in the single transistor and inverter is different. The research method of studying SET sensitive regions of the devices under FDSOI process was improved. The simulation result of inverter is more in line with the actual situation than single transistor, which will provide theoretical guidance for SET hardening.