Abstract: Because Silicon On Insulator Laterally Diffused Metal Oxide Semiconductor (SOI LDMOS） is manufactured on silicon film with a thickness of only tens to hundreds of nanometers. Under the action of high voltage and high current, the effect of Hot Carrier Injection (HCI) is more complicated, and the reliability of HCI is greatly challenged. In this paper, the hot carrier injection degradation mechanism of 15 V SOI LDMOS with two structures has been studied, and the characteristics of interface defects have been tested by the Charge Pumping method. When the HCI effect occurs in the channel region, the maximum channel transconductance degradation is obvious, and the saturation driving current degradation is relatively small. When the HCI effect occurs at the edge of the polycrystalline gate, the situation is just the opposite. The device structure and the distribution law of impact ionization rate are studied by TCAD simulation, and the influence mechanism of the negative charge generated by impact ionization on the drift region is found, and the damage mechanism of the SOI LDMOS device is revealed by HCI effect, that is, the position with the highest impact ionization rate. It provides an important experience reference for HCI reliability design and optimization of thin silicon film SOI LDMOS devices.