<p>When the subway projects cross the karst area, the unfavorable geology may bring many potential safety hazards to the subway design, construction and operation. There is a wide range of karst distribution in southwest China. Extremely irregular karst area is distributed in the strata along the subway lines. The Quaternary sediments are thick, which greatly increases the difficulty of detection. A multi-depth refined detection method of karst features under thick Quaternary sediments is proposed in this paper. Firstly, the karst cavities and fractures in the shallow stratum of the study area are determined by ground-penetrating radar (GPR). Secondly, the mid-deep resistivity cross-sections are delineated using surface electrical resistivity tomography (ERT) and transient electromagnetic method (TEM). The principal component analysis (PCA) is used to fuse the resistivity data. The three-dimensional imaging is established to delineate the distribution of karst anomalies. As the information obtained by PCA is more comprehensive, the accuracy of geophysical interpretation is improved. The multi-depth refined detection of karst under thick Quaternary sediments along subway lines is achieved through integrated analysis. This method has been applied in Changsha Metro Line 3 and achieved good practical results. It is of great application value for the karst tunnel treatment and the safe excavation.</p>