Organic thin-film transistors (OTFTs) with plastic substrates show good promise as signal amplification devices in flexible electronics. Reducing power consumption and improving device stability are two crucial requirements for the practical sensor application of OTFTs. Here, novel sandwich structured dielectrics with a polyelectrolyte as a core layer and polystyrene as two cover layers are developed for flexible OTFTs with decreased operating voltages, promoted carrier transport, reduced hysteresis, and improved air-stability. The flexible OTFTs with the sandwich dielectric deliver an improved mobility of 4.65 cm² V⁻¹ s⁻¹ at a low-operating voltage of −3 V, and exhibit good stability over continuous air exposure for one week and even for two months. The great improvement in performance of these OTFTs can be attributed to the high-capacitance, large leakage suppression, smooth surface, salient hydrophobic feature, and excellent interfacial compatibility with organic semiconductors of the sandwich structured dielectrics. By using the optimized sandwich dielectric, flexible OTFT pressure sensors are further achieved with an ultrahigh sensitivity of 691.9 kPa⁻¹ in a wide detection range at a low-voltage of −3 V. The integrated OTFT sensor array is demonstrated to display high-resolution spatial pressure mapping and motion route monitoring, suggesting its potential in smart sensing and intelligent recognition.