Abstract: The No.3 line of Nanjing Metro would cross a valley and karst area,which is one of the several major geotechnical problems during the construction.A preliminary survey showed that there were many caves in the area.It's located in the downtown area where has heavy traffic and dense buildings,many underground pipelines and complicated geological conditions.In order to find out the karst development situation in a certain place for the engineering,the high-density resistivity method was used for karst prospecting after combination of geological structure and comprehensive election.The WN measurement was carried out.The electrical sense points are located on asphalt or cement concrete pavements.The condition of the sense contact to the ground surface was poor.So it had substantial errors in the back analysis process.For this reason,the electrode with ground coupling conditions were improved in the measurement.The power supply voltage was also increased to improve the current penetration.During testing,the isolines were rendered on the test data directly.According to the characteristics of low resistivity distribution from the isolines,the karst development situation was inferred.The results coincide with the drilling results relatively.The results were successful.After the contrast analysis between field engineering geological investigation and the borehole data,the spatial distribution of the karsts was identified.The results show that the geophysical method has multiple solutions.Before the geophysical work,a field investigation should be carried out for the geological and tectonic conditions in study area.Then geological analysis of the karst characteristics should be carried out in order to guide the work of geophysical wiring and data interpretation well.When the ground conditions are poor,the methods of piling soil up and watering can be undertaken to solve the problem of electrode coupling.To the existence of error of the back analysis,the isoline map of apparent resistivity can be used directly to infer the locations of karsts.