The problem of large deformation of primary support during soft rock tunnel excavation is studied by taking the large crosssection loess tunnels on ZhengzhouXi′an Highspeed Railway and tunnels on ChengduKunming Railway in Tertiary Xigeda Fm with soft and weak surrounding rocks for examples. The causes of large settlement of soft rock tunnel primary support and its countermeasures are analyzed by theoretical calculation and site monitoring methods. The study results are drawn as follows: (1) The settlement of soft rock tunnel mostly exceeds 20 cm; and the main cause of large settlement of primary support is the overlarge pressure (897.4 kN in maximum) on arch foot, which significantly exceeds the bearing capacity of arch foot(arch springing). (2) The largest pressure of feetlock bolt is 55 kN, which occurs close to the steel frame; the pressurebearing property of large arch foot is significant and the extreme value is 0.9-1.7 MPa. (3) The methods of adding feetlock bolt (pipe), expanding arch foot and closing inverted arch timely are the keys to control the settlement of primary support of soft rock tunnel.