To meet the stringent requirements for high accuracy, wide dynamic range, and rapid response of pressure sensors under the complex operating conditions of high-speed train traction and braking systems, this study presents the design, fabrication, and multiphysics optimization of a high-performance piezoresistive pressure sensor chip. By emplo-ying multiphysics coupling modeling theory in combination with a structural parameter optimization approach, this study systematically investigates the synergistic influence of diaphragm thickness, piezoresistor layout, and doping concentration on sensor performance, and proposes a parameterization-based stiffness-sensitivity co-optimization strategy for the diaphragm. Furthermore, by developing an eight-mask photolithography process and a composite wet etching technique based on KOH / IPA, a submicron-level control accuracy of diaphragm thickness was achieved. Finite element simulation results demonstrate a sensitivity of 56.987 mV / kPa, a nonlinearity of 0.048% FS, and structural stability under 300% overpressure. This work addresses a key technological bottleneck in high-accuracy pressure sensor fabrication and lays the foundation for fully localized production of safety-critical sensing components in next-generation traction systems for high-speed train.