This paper briefly reviews the research and application progress of parallel seismic testing, focusing on the principle of detection of defects in pile shafts. It is considered that for piles under construction with an exposed top shaft, parallel seismic testing can provide a more comprehensive and reliable evaluation of the pile quality as compared with the conventional reflected wave method, because it has a shorter wave propagation path and lower interference, and can provide additional information on the quality of the pile shaft below the defect position and the location of the pile bottom. A case study was performed to show the application process of parallel seismic testing. The results show that measured signals of parallel seismic testing are clear and can reflect information on the quality of the pile part below the defect position.