An optimization theory is developed for the balanced symmetric superjunction with the interface isolator layer (I-SJ) in this paper. The theory includes two parts: the electric field calculation by the Taylor series method; the design formulas by the minimum specific on-resistance $R_{\mathrm{\scriptscriptstyle ON},\mathrm {min}}$ optimization. Based on the theory, a new I-SJ structure with a single cell is proposed, which shows the minimum $R_{{\mathrm{\scriptscriptstyle ON}}}$ among the superjunction (SJ) devices with the $1~\mu \text{m}$ shallow depths. $R_{\mathrm{\scriptscriptstyle ON}}$ of the new device is reduced by 53.5% compared with that of the conventional SJ lateral double diffused metal-oxide-semiconductor field effect transistor (LDMOS) and 67.6% with the conventional LDMOS under the same breakdown voltage $V_{B}$ of 658 V.