The utility model belongs to the technical field of testing equipment of a geotechnical engineering model, and discloses an automatic gas-liquid composite compensation loader. Three chambers of a loader cylinder barrel (2) sequentially are an upper chamber, a middle chamber and a lower chamber; air inlets connected with a gas pipeline are respectively formed in the upper chamber and the lower chamber (3); a free piston (3) is arranged at one end of the air inlet inside the upper chamber; a positioning ring (4) is arranged at one end inside the middle chamber filled with oil liquid; an oil inlet connected with a liquid pipeline is arranged at one end of the positioning ring (4); a piston rod head (9) of a work piston (5) at the other end inside the middle chamber passes through the lower chamber filled with gas and is fixedly connected with a loader pressure head (7); the other end of the loader pressure head (7) is connected with a model test specimen (8). By adopting the automatic gas-liquid composite compensation loader, rapid pressure compensation of an experimental device when the model is rapidly damaged can be ensured; a basically stable dead load at the boundary of the model is achieved; accurate simulation of the truth when rapid deformation and fracture are generated on the model is achieved.