The invention discloses a construction method for a reserved laterally widened structure of an underground box culvert structure. The method comprises the following steps: carrying out the future expansion on the side walls for the box culvert structure in advance, additionally arranging reinforced concrete pilasters to be used as vertical components for bearing horizontal water and earth pressure during foundation pits excavation in expansion; using top plates, middle plates and bottom plates to support the side walls of expansion sides, and arranging steel bar connectors in the plates. In later expansion, as the pilasters can be used as load-bearing components for lateral support of foundation pits, construction risk, construction difficulty and influence on the normal operation of an already built tunnel are greatly reduced, the construction is simpler and more convenient, and the construction period is shortened.