Karst is developed intensely in Heyetang viaduct.The crosshole electromagnetic wavepenetrating method has been used to detect type and scale of karst in Heyetang viaduct.The law of deep karst of the viaduct has been analysed according to achievements of drilling and crosshole electromagnetic wavepenetrating.Testing and effection have been discussed and evaluated.According to testing of drilling ,we have found that detecting precision of crosshole electromagnetic wavepenetrating is high. The comprehensive analyzing has made known that crosshole electromagnetic wavepenetrating method can be used to be an auxiliary mean on determining bearing layer of pile foundation.