In this paper, the thermal conductivity of lateral double diffused metal oxide semiconductor (LDMOS) was studied. In order to optimize their properties, the LDMOS device based on the lower surface of field (RESURF) theory join the second field plate technology. Power device self-heating effect will affect the carrier mobility, making its negative resistance effect in IV characteristic curve under the high-power condition. As the thermal conductivity of SiO2 is low, the self-heating effect of SOI device is more obvious. The simulation using Silvaco -TCAD software for different buried oxide (BOX) with different SOI layer thickness accordingly show that the thicker SOI layer and the thinner buried oxide layer, the smaller the self-heating effect.