Abstract：Research purposes： In November 17, 2013, Engineering Management Center of China Railway Corporation issued "the notice on carrying out karst exploration work of railway tunnel and subgrade under construction", which required carrying out the geophysical exploration in constructing railway tunnel and subgrade in karst area. In the past 3 years, we have carried out a large number of geophysical exploration work in the construction of Guiyang-Guangzhou railway line and other lines under construction, and made some effect, but there are also many problems, for example the geophysical results cannot meet the design requirement of karst disease treatment. This paper studies the existing problems, which will provide basis for correctly positioning the geophysical work and improving detection scheme of karst tunnel and subgrade in the future.  Research conclusions： (1) Because the site is difficult to have the prerequisite of geophysical exploration, the limitations of the selected geophysical methods and using geophysical exploration can not directly detect underground fissure water and other reasons, geophysical exploration of the railway tunnel and subgrade base under construction in karst area can only solve part of karst detection problems of the tunnel bottom and subgrade base. (2) The geophysical exploration belongs to the indirect exploration means, and in order to exclude multiple solutions of the geophysical exploration results, we should adopt comprehensive exploration methods in karst area, namely 30% geophysical anomaly excavation revealed or confirmatory drilling. (3) When validation data show that the karst is very developing, we should choose the cross-hole electromagnetic wave (seismic wave) tomography method to fine detection of karst, which provides reliable basic information for the subsequent design of tunnel and subgrade karst. (4) The research conclusion is applied on karst exploration in the railway tunnel and subgrade.