Abstract: To address the challenges posed by small acoustic impedance differences, weak interface reflection signals, and poor identification capability in double-layer metal composite structures composed of carbon steel and stainless steel, this study proposes a Barker code-modulated excitation method. Simulation-based thickness measurement is conducted on such composites with small interfacial acoustic impedance mismatches. Three excitation signals—namely, a pure sine wave, a Barker code-modulated sine wave, and a windowed Barker code-modulated sine wave—are designed. A stability comparison under noisy conditions is performed, and the optimal signal is selected for thickness measurement simulation. To enhance interface reflection waves, local mesh refinement and high-frequency excitation are employed in the simulations. In post-processing, matched filtering and localized amplification are applied to improve echo recognition. Results show that the 13-bit windowed Barker code signal exhibits superior robustness against multiple noise disturbances. Stable echo recognition and accurate thickness estimation are achieved across various thickness configurations. This work investigates ultrasonic propagation characteristics and thickness measurement procedures in weak-echo interfacial environments, providing a feasible approach for ultrasonic testing of such composite structures.