The present invention relates to the field of tunnel excavation, in particular to a method for expanding tunnels, including the construction methods for tunnels in Class IV and V surrounding rocks, and the construction methods for tunnels in Class III surrounding rocks, wherein the construction methods for tunnels in Class IV and V surrounding rocks include The construction steps are as follows: divide the excavated tunnel, including Part I in the middle of the vault, Part II on the left side of Part I, Part III on the right side of Part I, Part IV on the lower side of Part II, and Part V on the lower part of Part III and Part VI of the lower part of I. This construction method adopts double-side wall excavation in sections, and the excavation method of the arch first and the wall later has the following advantages: the vault sinks the least, the horizontal displacement is small, the axial force of the temporary support is the smallest, and the support is reliable, and the project progress is fast . Moreover, this scheme is very suitable for the expansion and excavation construction of ultra-large-span, ultra-large-section, and small-spacing tunnels. The construction safety is high and there will be no collapse.