This paper focuses on the influence of regenerator on the performance of a single-stage looped thermoacoustic engine. Numerical simulation has firstly been carried out to investigate how the regenerator efficiency, generated acoustic power, and acoustic field in the regenerator are affected by the cross-sectional area, regenerator length, and mesh number. Based on the numerical optimization, an experimental setup with a normalized cross-sectional area of 14.5 and a normalized regenerator length of 0.0036 has been established, and the influence of the hydraulic radius of regenerator (determined by regenerator mesh number) on the onset process and the stable oscillation of the system has been studied. The results show that the ratio of the hydraulic radius of regenerator over thermal penetration depth should be in the range of 0.2 to 0.3, in order to achieve a low onset temperature difference and a high thermal efficiency. In the experiments, an onset temperature difference of 84 degrees C and a thermal efficiency above 4% at a temperature difference of about 200 degrees C were obtained, showing the potential to recover low-grade heat.