The total-ionizing-dose radiation effect in high voltage SOI pLDMOS devices was studied. The degradation mechanism of breakdown voltage under different bias conditions was analyzed, fixed trap charges were introduced at the interface of different oxide layers using TCAD， and the effect of the total ionizing radiation dose was simulated. The results showed that the total dose radiation introduced radiation trap charges QBOX and QFOX at FOX and BOX. The QFOX elevated the transverse electric field near the drain and reduced the buried oxide electric field, shifting the breakdown location from the body to the surface, and leading to degradation of the breakdown voltage. The QBOX reduced the buried oxide electric field and reduced the buried oxide voltage drop, leading to degradation of the breakdown voltage.