A looped three-stage thermoacoustically-driven cryocooler system is introduced. Based on classic thermoacoustic theory, simulations are performed to investigate the effects of three representative coupling positions (inlet, middle, and outlet) of the resonance tube. The total exergy efficiency is found to depend on the dimensions of the resonance tube, demonstrating the importance of this parameter. For the same resonance tube length, the highest exergy efficiency of 16.3% is achieved for the outlet coupling position, whereas the middle and inlet coupling positions only achieved highest exergy efficiencies of 9% and 14.93%, respectively. The distribution of the phase difference, acoustic power, and exergy loss ratios of the main components are then presented to clarify the coupling mechanism. The results show that better phase distribution in the regenerator and less exergy loss in the resonance tube contribute significantly to the superior performance of the outlet coupling position.