Abstract: Engineering excavation and construction lead to unloading damage of soft rock, and its degradation characteristics under water-rock interaction are closely related to the degree of unloading damage. Taking the red mudstone in Badong area as the research object, the simulation tests on the unloading damage of mudstone are designed and carried out, and then the water-rock interaction tests are carried out on the mudstone samples. The results show that the unloading damage leads to the destruction of the local mineral particle morphology of mudstone and the development of microcracks. Under the subsequent water-rock interaction, the degree of cementation between particles continues to weaken, and the pores and fractures further expand. The macro- and meso-parameters increase with the time of water-rock interaction, in which the mass (m) first rises, then drops suddenly and then tends to be stable, the longitudinal wave velocity (P) first decreases and then rises, and the volume strain \text(\varepsilon _\textv\text) and the fractal dimension (K) increase gradually. The growth rate of each parameter increases exponentially with the increase of unloading damage degree (D). When D ≥ 0.49, the growth rate of each parameter increases sharply. The quality, longitudinal wave velocity, volume strain and fractal dimension are determined as the variables of the reaction system with water, and the nonlinear dynamics theory is used to establish the degradation model for mudstone under water-rock interaction, and the rationality of the model is verified. The results indicate that the proposed model can provide a theoretical reference for characterizing the change trend of macro- and micro-characteristic parameters of unloading damaged mudstone under water-rock interaction.