This study examined the influence of an in-line phase-adjuster on the energy conversion efficiency of a thermoacoustic Stirling heat engine (TASHE). The numerical and experimental investigations were performed. An acoustic field in the system can be adjusted using the phase-adjuster. Therefore, the thermoacoustic engine could maintain high-level performance at all operating conditions. The engine is composed of a torus section where core components were located, and a long resonator pipe. The phase-adjuster or the telescopic in-line piston was set up at the tail of the resonance tube. The TASHE was modeled by employing DeltaEC to search the optimal configurations of the prototype. Due to the variations of acoustic loads or operating conditions from the design criteria, the engine absolutely cannot maintain maximum efficiency. The proposed phase-adjuster could bring back its maximum efficiency by re-matching the acoustic impedance in the regenerator. The TASHE was designed to operate with compressed air at 9 bar. In the experiments, the self-excited temperature of the engine was around 480 °C, and the steady-state temperature was about 397 °C. The TASHE can provide an acoustic power of up to 40 W. The thermo-to-acoustic efficiency of 12.03% related to 22.56% of the theoretical Carnot efficiency was achieved. There was a reasonably good agreement between the measured and DeltaEC simulation results. This can reflect on the preciseness of the proposed model. Furthermore, the function of the phase-adjuster in tuning the acoustic impedance was also demonstrated experimentally. In case of the TASHE being operated under the off-design conditions, i.e., due to pressure leak or heat losses, these scenarios would drop the efficiency of the system. The research presented in this study can confirm that the phase-adjuster is the component applied to tune the acoustic field in the regenerator accurately with minimum changes in the system. It could help to improve the efficiency of such consequences.