Cement mixing piles are common method of treating soft soil foundation in coastal areas. However, there are some problems in practical engineering, such as cutting corners and unstable pile quality. Current methods for detecting mixed piles are high cost, have a low sampling-ratio and can permanently damage the integrity of the pile body. Non-destructive and accurate detection methods are urgently needed. In this paper, three methods of time-lapse cross-hole full waveform inversion (separate inversion, continuous inversion and double difference inversion) are proposed by combining cross-hole seismic, full waveform inversion and time-lapse seismic exploration. These methods are applied to the expansion project of Singapore Changi Airport, and the detection effects of various methods are analyzed and compared. The test results show that the cross-hole wave velocity test is easily disturbed by the environment and it is difficult to reflect the actual range of the pile foundation. The background field has a certain influence on the resolution of full waveform inversion. Continuous inversion can effectively highlight the location and scope of the pile foundation and double difference inversion can further weaken the influence of background field on the basis of continuous inversion, so as to improve the imaging accuracy.