The invention discloses a lower dam body with gravel sand through throwing and a construction method for increasing the relative density of the dam body. The dam body comprises an upstream dam toe and a downstream dam toe. The space between the dam toes is filled with the formed gravel sand dam body in a thrown manner. The gravel sand dam body comprises a gravel sand dam main body below the water surface and a gravel sand dam main body above the water surface. A plurality of rows of vibroflotation compaction holes are drilled in the dam top of the gravel sand dam main body above the water surface. Every two close rows of vibroflotation compaction holes are distributed in a staggered manner. Each row of vibroflotation compaction holes are formed by first-sequence vibroflotation compaction holes and second-sequence vibroflotation compaction holes alternately. Reasonable hole distribution, hole manufacturing, vibroflotation compaction and continuous filling are performed on the dam main body, with water filled with grave sand above the water surface through throwing, of a low dam, and the relative density of the dam body with water filled with grave sand through throwing is increased and not smaller than 0.75 regulated in design standards of a current rolled earth-rock dam so that dam body deformation can be controlled and stability and safety of the dam body can be guaranteed.