The developing characteristics of karst cannot be cleared by traditional investigation methods. Hence, the investigation work of karst of Kunming Rail Transit Line 4 is taken for example to introduce the preliminary and detailed investigations of geotechnical engineering. And then the engineering geological drilling and field geophysical testing (cross-hole tomography(CT), high density electrical method and geological radar) are taken as the main investigation methods, and engineering geological surveying, in-situ testing, borehole pumping test and indoor geotechnical testing, etc. are adopted as well to learn the distribution, scale and shape of karst along the line. And a quantitative or qualitative evaluation is carried out. The results can provide reliable geological basis for the design of karst special treatment construction drawings, optimization of design schemes and improvement of construction organization design.