Abstract: The Xiushan tunnel of Yuxi to Mengzi railway geological condition is complex.The whole tunnel make use of the controlled source audio frequency magnetotelluric(CSAMT) method to explore formation lithology,geological structure,karst features,groundwater characteristics,coal bearing formation.Through comparative analysis of the construction,it is found that CSAMT method can more accurately interpret development anomaly zones in the tunnel region,such as the fault zone,rich water zones,etc,and it played a good role for design and guide the construction.