The invention discloses a construction method for preparing foamed light soil in site in an integrated form to fill a substructure of a high-speed rail. The construction method mainly comprises the following steps of building a planetary cement paste blending unit; building a mobile light soil preparation station; arranging spot conveying pipes; foaming a mixing and delivering pump; blending the foamed light soil; delivering the foamed light soil to a filling field; filling, pouring and smashing the foamed light soil in site; and maintaining the foamed light soil. Full automatic foamed light soil equipment including a planetary stirring station and a paste preparing, foaming, mixing and delivering station which form a complete set; multiple steps of the foaming technology and the mixing technology are carried out, and linkage automatic control is realized; the density and the flow of foam and the density and the flow of the light soil are automatically controlled in real time; high productivity is also realized; the productivity of single equipment is higher than 90m(3)/h; and the construction progress, the construction safety and the engineering quality can be ensured.