Based on the research of controlled source audio-frequency magnetotelluric sounding and NLCG inversion method in this paper, The author analyzed the electrical constraint inversion method which used drilling, logging, geology, seismic, high density resistivity method as a kind of priori information,and combined the measured CSAMT data of a tunnel in the BAOLAN railway, contrasted and analyzed the result which used or not used the electrical constraint inversion, finded the constraint inversion results had a higher accuracy in finding fault, weathering, lithologic and determineing the boundary thickness.