Abstract: Crack detection is crucial for ensuring the integrity and safety of critical components and structures in industries such as aerospace, oil and gas, and rail transportation. Regular crack detection is required in these fields to prevent severe accidents like failures, leaks, or explosions. This study investigates a terahertz coaxial probe-based non-destructive crack detection method, exploring the impacts of coupling gap, lift-off height, and tip geometry on detection sensitivity through 3D electromagnetic field simulations, while further examining the feasibility of subsurface crack detection. Results demonstrate, Coupling gap and lift-off height significantly affect detection sensitivity by modulating near-field energy distribution. After parameter optimization, sensitivity increased by 46.7% compared to conventional low-frequency probes; Chamfered inner conductor tip geometry enhances edge field focusing effects, enabling detection of 0.03 mm micro-cracks; The probe successfully identifies cracks beneath non-metallic coatings (0.035 mm depth, 0.2 mm width), maintaining detection capability even with surface coatings ≤70 μm thick. The dielectric properties and thickness of coatings substantially influence detection performance, providing valuable insights for high-precision non-destructive detection of subsurface defects.