This paper presents a weathering test apparatus developed for testing the deformation and shear strength behavior of rockfill materials for high dams under coupling actions of drying/wetting cycles, cooling/heating cycles, and vertical loads. The apparatus was integrated with a vertical load unit, a lateral shear unit, a drying/wetting circulation unit, and a cooling/heating circulation unit. A cell was designed to perform direct shear tests under normal pressures up to 5.5 MPa after long-term weathering associated with wide variations in temperature and moisture content. It accommodates specimens 150 mm in diameter and 150 mm in height. The related experimental procedures and calibration of the device are discussed. Results of tests performed on an argillaceous siltstone used as the rockfill material for a 261.5 m high dam are presented in order to highlight the features of the apparatus and the weathering effects on soft rock.