High-speed railway are boosting in China and many railway routes have to go though mountainous area which has complex geotectonic setting. Mapping the deep buried long railway tunnel comes to prove difficulty in the railway route investigation. A successful case history of applying the highfrequency electromagnetic (HFEM) method to investigate Taihangshan (THS) Tunnel route is presented in this paper. The high-frequency EM system (EH-4, with frequency range from 100 KHz to 12.6Hz) is used for the data acquisition. The orthogonal components of electromagnetic field are measured in high frequency EM method. TSH Tunnel, located in north China, has a length of more than 27km and a bury depth of up to 450m. Up to now, it is the longest railway tunnel in Asia. Most of the tunnel goes through the Karst rocks, coal measure strata and gypsum breccia, where the engineering geologic condition are very complex. High frequency EM was introduced to the investigation to help mapping the wall rock, concealed fault and the gypsum breccia layer along the tunnel route. Most of the anomalies provide HFEM has been verified by drilling or other methods. HFEM could get acceptable result in the deep long buried tunnel investigation.