The deformation control of the core breakthrough section of soft rock mountaincrossing tunnel with extremely high ground stress is very difficult. The field test is carried out and the data collected are analyzed by taking Muzhailing Tunnel on LanzhouChongqing Railway for example. Some conclusions are drawn as follows: 1) The length of the core breakthrough section crossing mountain is determined. 2) The comprehensive deformation control scheme of “fourlayer tunnel primary support structure + radial grouting + long anchor bolt + long anchor rope” [JP2]for core breakthrough section crossing mountain is proposed. 3) The deformation control effect of multilayer support structure of the breakthrough section is good. 4) The construction scheme and deformation control scheme for breakthrough section are rational.