The invention discloses a construction method of a coral sand foundation expanding cement mortar injection pile. A mortar solid discrete material pile is arranged on the periphery of a pile body, a small-diameter mortar injection pipe is arranged in the center of the pile body, and expanding mortar enters a discrete material through the mortar injection pipe; the peripheral discrete material is glued and solidified into a whole through expanding cement mortar; meanwhile, the expanding cement mortar enters a coral sand foundation under the action of mortar injection pressure, and the character of the expanding cement mortar can be improved; and after the expanding cement mortar injection pile is formed, the pile body expands, a soil body around the pile can be subjected to compaction, the character of soil around the pile is improved, and the strength of the soil around the pile is improved. According to the construction method, a construction process is clear, the construction speed is high, the pile forming effect is good, bearing force is high, the manufacturing cost is low, the geological resource of calcareous sand is fully utilized, the environment is protected, and energy saving and environment protection are achieved. The construction method has positive effects on reclamation project work of the large-area coral sand foundation in the South China Sea, the problems about pile forming and the like on a calcareous sand foundation can be solved well, and therefore civil purposes, military purposes and multiple other purposes are met.