Abstract: Laboratory weathering tests on a loaded argillaceous siltstone rockfill are performed by use of a newly developed test apparatus. The degrading mechanism of rockfill particles and the degrading strain of rockfill materials under the coupled wetting-cooling and drying-heating cycles are investigated. The test results show that the coupled wetting-cooling and drying-heating cycles can induce a remarkable degrading strain for rockfill materials, therefore the degrading strain of rockfill materials owing to variation of natural environment conditions should be a main source of long-term deformation of a high rockfill dam. Based on the test results, a constitutive model for rockfill degrading with four parameters is established, which can be applied to the finite element analysis by the initial strain method.