The invention relates to a method for construction of a new and old roadbed light-soil splicing broadening structure based on a fabricated formwork. The method mainly comprises the following steps of construction preparing, embankment pile arranging, on-site poured sash beam and boundary beam construction, T-shaped anti-shear part and jacking installation, prefabricated side slope plate installation, steel bar net piece laying, light soil refilling, new road surface layer construction and green operation in side slope sash beam. According to the method for the construction of the new and old roadbed light-soil splicing broadening structure based on the fabricated formwork, juncture of new and old roadbeds of the embankment broadening structure is high in anti-shear capacity and good in stability.