A continuous and analytical surface potential based model for SOI LDMOS, which has considered the automatic transition between fully and partially-depleted statuses, is present. Front and back surface potentials are obtained analytically through solving the surface potential equation by adopting the accurate algorithm used in PSP. Some key variables in PSP have been modified under the fully-depleted status. The good agreements between simulated and measured dates indicate that this model can predict the DC characteristics of SOI LDMOS accurately.