The invention discloses an underground buoyancy model testing device. The device comprises a chamber body, a measuring structure and a testing structure, wherein the upper end of the chamber body is opened; a measuring structure fixing component is also arranged at the upper portion of the chamber body; one end of the measuring structure is fixed to the lower portion of the measuring structure fixing component; the other end of the measuring structure is connected with one end of the testing structure which is positioned inside the chamber body; the lower position of the sidewall of the chamber body is provided with a through hole; the through hole is provided with a water pipe which extends into the chamber body; soil layers which are uniformly distributed are arranged between the chamberbody and the testing structure which is positioned inside the chamber body; the water pipe is positioned below the soil layers; and a position, starting from the soil layers, of the chamber body is upwards provided with scales. The underground buoyancy model testing device disclosed by the invention has the advantages that the testing structure is installed and the water pipe is used for chargingwater, and the measuring structure is used for checking the relationship between the water level and the height of the soil layers, and finally the measurement on the relationship between the water level and the buoyancy in a soil body is realized.