Abstract: The stress relaxation properties of soft rock are an important aspect of its long-term mechanical properties, which are very important to keep soft rock tunnel long-term stability. The triaxial stress relaxation tests on argillaceous siltstone are carried out under confining pressures from 15 to 35 MPa. The relaxation curves can be divided into two stages: the transient relaxation and steady relaxation. When the stress relaxation starts with the initial deviatoric stress, which is much lower than the peak stress of the specimen, the relaxation is mainly composed of transient relaxation. However, when the initial deviatoric stress approaches to the peak stress of the specimen, obvious steady relaxation is observed. The comparisons of relaxation curves obtained from specimens under different confining pressures show that when the initial deviatoric stress is much lower than the peak stress of the specimen, the confining pressure has little effect on the relaxation behaviour of the specimen; and when the initial deviatoric stress approaches to the peak stress of the specimen, the relaxation stress decreases with the increase of the confining pressure. Based on the analysis on the results of triaxial relaxation tests, the Nishihara model is employed to describe the relaxation behaviour of argillaceous siltstone, and the comparisons between numerical and measured relaxation stress indicate the rationality of the Nishihara model.