The invention relates to a construction method of pouring a retailing backwall using foam light soil in a highway reconstruction and extension project. Firstly a base is cleaned and compacted, a layerof broken stones with the thickness of 30 cm is laid, steps are arranged along the route driving direction and along one side, close to a road center line, of the cross section direction, a convex step is poured by using a form hanger on the side close to a side slope, and the steps are arranged for better lapping of the foam light soil and subgrade soil. Waterproof geomembranes are laid on the lowest layer and the topmost layer of a foam light soil structure, and earthwork standard rooms are laid on the positions which are respectively 50 cm from the bottom layer and 50 cm from the top layerto enhance the overall strength. By adopting the construction method, under the condition that the area at the retailing backwall is small and is not favorable for construction of a large-scale instrument, the construction can be simplified, the operation is convenient, by utilizing the features of quick early strength formation and good integrality of the foam light soil, the dependency on lateral wrapping soil is reduced, and template supports are used instead for pouring. The construction method has a relatively good effect on reducing bump at bridge-head.