The newly-built Sichuan-Tibet Railway passes through the Western Sichuan Plateau. Precipitous topography， vulnerable ecology and poor traffic conditions of the region bring about many obstacles for carrying out investigation drilling， especially for lots of extra-long deep tunnels. Horizontal core drilling is one of the important means for investigation for tunnels. In view of these conditions， the original design of the portable drilling rig EP600Plus will face such problems as inability to drill horizontal boreholes， high drilling resistance， low coring efficiency due to frequent high-pressure water gushing. With flexible application of various methods including drill upgrading， process innovation， supply of auxiliary equipment， and building access bridges and roads with steel pipes for the first time as environment protection measures，use wire-line core drilling technology， a horizontal coring borehole was completed at depth of 903.28m with a full technical plan established and proven with application. The main achievements can provide important reference for similar investigation projects.