In this work, a silicon-on-insulator (SOI) lateral diffused MOSFET (LDMOS) incorporated with sidewall field plate (SEP) is presented and investigated using three-dimensional numeric simulation. A new additional electric field peak is formed, due to the compound field plate established along the n-drift region. The lateral surface electric field can be modulated, which enhances the breakdown voltage. Meanwhile, the doping concentration of n-drift region is increased, which decreases the specific on-resistance. Compared with the superjunction LDMOS, a 23.7% increase in the breakdown voltage and a 21.7% decrease in the specific on-resistance are obtained in the SFP-LDMOS device.