High-frequency ultrasonic transducers are pivotal for detecting minute defects, offering distinct advantages in terms of non-destructive evaluation, non-invasiveness, and superior spatial resolution. However, achieving effective focusing and efficient acoustic transmission for ultra-high-frequency ultrasonic transducers is a significant challenge. To address this challenge, a mass–spring acoustic matching layer is designed for a transducer based on a half-concave LiNbO₃ piezoelectric element at 100 MHz. The proposed mass–spring stack, comprising a 0.25 μm Au layer and a 2.5 μm Parylene-C layer, operates within a deposition-friendly thickness range ideal for curved substrates, while a conventional quarter-wavelength Parylene-C layer would necessitate a thickness of 6.0 μm at this frequency. The transducer is modeled in COMSOL Multiphysics 6.1, coupling solid mechanics, electrostatics with piezoelectric effects, and pressure acoustics for the water load. Using a fixed concave geometry (curvature radius is 1 mm, which means the focal length is also 1 mm), a frequency sweep from 50 MHz to 150 MHz is conducted to evaluate performance. Analysis of the pressure distribution in the focal plane reveals that the focal length and the −6 dB beamwidth are predominantly governed by aperture diffraction and exhibit minimal variation upon incorporation of a matching layer. The focal length is approximately 1.02 mm within the excitation frequency range of 70 MHz–150 MHz, and the beamwidth decreases markedly with increasing frequency. At 100 MHz, the measured −6 dB beamwidth is approximately 31.6 μm without a matching layer, 32.3 μm with a quarter-wavelength matching layer, and 31.0 μm with a mass–spring matching layer. Crucially, quantitative comparisons reveal that this design yields a systematically higher focal pressure; near the 100 MHz design frequency, the acoustic pressure amplitude achieved with the mass–spring configuration is 1.5 times greater than that obtained using the quarter-wavelength reference. These research results provide a theoretical basis for the application of ultra-high-frequency ultrasound in the detection of tiny defects.