The invention relates to a support device for floorstand-densified cultivation of open-field watermelons. The support device is characterized by comprising a base, an upright rod, at least one buried rod, two supporting rods and connecting members, wherein the base is of a closed bracket which is formed through sequentially connecting at least three rod bodies, which are located in the same plane, end to end; the upright rod is arranged in a manner of being vertical to a plane where the base is located, and the bottom end of the upright rod is fixedly connected with the base; the at least one buried rod are arranged in a manner of being vertical to the plane where the base is located, and top ends of the buried rods are fixedly connected with the base; the two supporting rods are symmetrically distributed at the outer side of the top end of the upright rod; and the connecting members are arranged at the outer side of the top end of the upright rod or/and the outer side of the supporting rods. The support device can be used for achieving the vertical cultivation of open-field small-sized watermelons and has the advantages of high land and space utilization ratio, high planting density and light diseases and insect pest occurrence, the planting density of the small-sized watermelons is effectively improved, the leaf area of vines is enlarged, the photosynthetic efficiency is increased, the incidence rate of diseases and insect pests of the small-sized watermelons is lowered, and thus, the production increase and income increase of the open-field small-sized watermelons are achieved.