The invention discloses a widening subgrade structure based on an existing railway subgrade. The widening subgrade structure comprises a step arranged on the side slope of the existing railway subgrade, an inverted filter arranged on the step, and a retaining wall poured on the outer side of a widening railway embankment. The space between the inverted filter and the retaining wall is filled with light soil. A crack-prevention steel bar mesh is laid at the preset height in the light soil. A subgrade filler is constructed on the light soil. The invention further provides a widening subgrade method of the existing railway subgrade. According to the subgrade widening structure based on the existing railway subgrade, the load of the widening subgrade can be reduced greatly through the characteristic of small density of the light soil, the stability of the widening subgrade is improved, and extra sedimentation of a railway operation subgrade is reduced. Through the retaining wall on the outer side of the light soil, the usage quantity of the light soil can be reduced, construction land is saved, and the retaining wall can serve as a template for construction of the light soil.