Under the guiding of New Austrian Tunneling Method, there are two basic construction methods, i.e., CRD construction method and both side drift method, for large crosssection tunnels in weak strata. The excessive settlement of the primary support is a serious problem in the construction of large crosssection tunnels, which causes the projection of the primary support structure into the tunnel clearance and thus the replacement of the primary support structure.  Because of the excessive settlement, serious collapse accidents occur one after another. For the moment, all the traditional construction methods can not effectively solve the problem. A new tunnel excavation method, i.e., bottom heading method, is recommended, which can not only completely solve the problem of primary support settlement and convergence, but also can guarantee the construction safety, improve the construction speed and construction quality and reduce the construction cost.