Although the ultrasonic testing technology provides the high-precision advantages in pipeline integrity assessment, its dependence on the liquid coupling severely limits the application in the natural gas pipeline environments. To address the challenge of ultrasonic energy transmission difficulties caused by the severe acoustic impedance mismatch at the gas-solid interface of high-pressure natural gas pipelines, this study proposes a high-pressure gas-coupled ultrasonic testing method based on the acoustic resonance principle. By integrating the Redlich-Kwong equation of state with ultrasonic wave propagation theory in multilayer media, a theoretical model for the ultrasonic resonant reflection under the high-pressure conditions was established. The model incorporates a complex wavenumber to characterize the acoustic attenuation and systematically reveal the influence mechanism of pressure on gas acoustic properties as well as resonant response, and the analytical expression for the resonant frequency shift caused by medium damping is also derived. Then the coupling effects of pressure, propagation distance, wall thickness, and resonant order on the resonant frequency were quantified with the simulation analysis. Furthermore a high-pressure experimental platform was established, and the experimental validation was performed using nitrogen as an equivalent medium. Simulation and experimental results demonstrate that the gas acoustic impedance significantly improves when the pressure increases to 1 MPa. Specifically the amplitude of the received resonant signal is enhanced by approximately 69 dB while consistently exciting multi-order resonant modes when the pressure reaches 3 MPa. Under such conditions, the thickness measurement error of steel plates ranging from 6.2 to 8.2 mm remains below 0.15 mm, whose resonant signal fluctuation amplitudes are smaller than 1 V within the 40~100 mm propagation distance range, demonstrating the good adaptability to variations in lift-off distance. The study verifies the accuracy and reliability of acoustic resonance for the wall-thickness measurement of high-pressure natural gas pipelines, thereby overcoming the precision limitations of conventional pulse-echo method in the high-pressure gas environments and providing a new theoretical foundation as well as experimental support for the nondestructive testing of transmission pipelines.