Abstract: The softening mechanism of soft rock is essentially the deterioration of microstructure caused by change of water-rock interface. The research on the water-rock interface is of great importance. Considering the general characteristics of the soft rock, from the idea of the inverse process of cemented diagenesis, the chemical element analysis of silty mudstone soak solution with different immersion time is carried out. The evolution process of microstructure of the soft rock is studied by using the polarized light micro-section. The concept of interfacial cemented bonding structure is proposed to investigate the established model for evolution of water-rock interface. Based on the diffusion theory, the theoretical equation describing the evolution of water-rock interface is derived and compared with the experimental results. The meso-softening damage factor of soft rock is proposed and deduced, and introduced into block discrete element simulation. The results show that the softening process of the soft rock is accompanied by the shedding and suspension of particles and the dissolution of soluble substances. Besides, the softening of the soft rock has obvious nonlinear dynamic characteristics. The fitting degree between the calculated values and the experimental results is relatively high, which verifies the reliability and rationality of the theoretical equation. The numerical results are in good agreement with the experimental ones.