The utility model relates to the technical field of mechanical test devices, in particular to a hydrostatic buoyancy model test device for an underground structure in a sandy soil foundation. The device comprises an outer box and an inner box, and the inner box is located in the outer box; displacement sensors are arranged at four right angles of the upper end of the inner box respectively, a strain gauge is arranged in the center of the bottom of the inner box and connected with the lower end of a digital display pull-push dynamometer through a straight rod, the upper end of the digital display pull-push dynamometer is connected with a rod body of a supporting rod, and the supporting rod is parallel to the longitudinal center line of the outer box and can slide along the upper edge of anouter box transverse frame; the center of the bottom of the outer box is communicated with an external water tank through a pipeline, a water pump is arranged at the water inlet end of the pipeline, sheet-shaped sponge is arranged at the position, corresponding to the water outlet end of the pipeline, of the center of the bottom of the outer box, and the area of the sheet-shaped sponge accounts for 70%-80% of that of the bottom of the outer box. The buffering mechanism is adopted, water flow can evenly and slowly pass through sandy soil during water inflow, and the situation that when the water flow makes contact with the sandy soil, a soil layer is scoured due to the too high speed, and small-range piping is formed is avoided.