Conventional super-junction lateral double diffused MOSFET (SJ-LDMOS) fabricated on Silicon on Insulator (SOI) substrate suffers from low breakdown voltage under the same on-resistance due to substrate-assisted depletion effect. To suppress this effect, it is important to find the charge density in the inversion layer under buried oxide. In this paper, we propose a charge density equation and its formulation. The results were used in a 3D device simulator to optimize the device structure. The experimental results confirm that the equation is useful to optimize device performance. The breakdown voltage of structure increased 54% and on-state-resistance decreased 58% compared to conventional SJ device. The device fabrication procedure is fully compatible with mainstream SOI CMOS process.