Widening structure for retaining wall section of mountain highway and construction method

The invention discloses a widening structure for a retaining wall section of a mountain highway and a construction method. The widening structure comprises a first water draining cushion layer which is arranged outisde a retaining wall of an old highway, a second water draining cushion layer and a second water draining cushion layer, wherein the second water draining cushion layer and the third water draining cushion layer are sequentially upwards arranged along the retaining wall of the old road; the first water draining cushion layer, the second water draining cushion layer and the third water draining cushion layer form an L-shaped water draining cushion layer; a stone filled subgrade layer and a stone filled substrate transition layer are sequentially paved in the water draining cushion from bottom to top; a soil subgrade filler layer, a roadbed and a pavement are sequentially paved above the stone filled subgrade transition layer; steps are arranged at parts where the old subgradeis joined with the soil