Hutiaoxia underground interchange project of Xiangli expressway is one of the key control projects of Shangrila to Lijiang highway. It is the mountain underground interchange tunnel with the largest cross section in China at present. The maximum excavation span in the convex part of the superlarge crosssection tunnel of the project is 28.96 m, and the dome is flat, which easily causes the surrounding rock to relax, lose stability and collapse during construction. In order to meet the requirements of rapid construction and safety, the construction methods are studied. The side arch foot of the leading tunnel of the multiarch tunnel is designed as an enlarged arch foot, and the twolayer support structure is changed into three layers. To strengthen the support of the surrounding rock of the superlarge crosssection tunnel, the twostep core retaining soil method is adopted to excavate after grouting. Compared with the original doubleside wall pit guiding method, the excavation efficiency is improved nearly three times. The analysis of monitoring data shows that this method is effective, and the purpose and requirements of the construction are achieved.