In this study, we fabricated the dual gate (DG) ion-sensitive field-effect-transistor (ISFET) with flexible polyimide (PI) extended gate (EG). The DG ISFETs significantly enhanced the sensitivity of pH in electrolytes from 60 mV/pH to 1152.17 mV/pH and effectively improved the drift and hysteresis phenomenon. This is attributed to the capacitive coupling effect between top gate and bottom gate insulators of the channel in silicon-on-transistor(SOI) metal-oxide-semiconductor (MOS) FETs. Accordingly, it is expected that the PI-EG based DG-ISFETs is promising technology for high-performance flexible biosensor applications.