Abstract: In order to explore the deterioration mechanism of water-rock-stress relaxation coupling,a self-deve￣loped water-rock coupling test device was used on the RMT-150C rock mechanics test platform. Water-rock-stress relaxation characteristics tests and uniaxial compression failure tests under different relaxation strain levels were carried out. Combined with nuclear magnetic resonance tests,SEM electron microscope scanning,and solution parameter tests,the deterioration law of macro-fine-microstructure characteristics of mudstone under different damage states was analyzed. The results show that: (1)The stress relaxation of mudstone under water-rock coupling is typical incomplete attenuation relaxation,including three stages: rapid,deceleration,and stable relaxation. (2)In the process of water-rock-stress relaxation,the water-rock interaction plays a leading role in the increase of the number of micro-pores,and stress relaxation causes the micro-pores to develop rapidly and connect to medium and large pores. The water-rock-stress relaxation effect leads to the transformation of the meso-structure of mudstone from smooth,dense,and few holes and fissures to uneven,loose,and developed holes and fissures. (3)During the water-rock-stress relaxation process,the pH value,TDS value,and ion concentration of the soaking solution initially increased and then stabilized with time,indicating the water-rock-stress relaxation coupling effect. In the early stage,the hydration reaction was severe,and the mineral particles were dissolved and hydrolyzed to a high degree. Subsequently,the hydration reaction slowed down into a stable state. (4)In the process of water-rock-stress relaxation,mudstone degradation is progressive and interacts at the micro-fine-macro scale. Therefore,a macro-fine-micro deterioration mechanism is established.