Abstract: This paper briefly introduces the geophysical analysis of Sanjiacun tunnel in Changning county of Baoshan city by using EH4 magnetotelluric method. High frequency magnetotelluric method is used to find out the integrity and the grade of surrounding rocks, evolution of karst and fault structures in this section along the longitudinal axis of the tunnel,so as to provide necessary scientific basis for assessment of tunnel engineering geological conditions. In this work,one geophysical section has been completed, basically making clear of the evolution and distribution of stratigraphy,structure and karst in this portion.