Hydrochemical action accelerates the dissolution of calcareous-cemented rock and the degradation of microstructure. This leads to the deterioration of macroscopic mechanical properties and increases the risk of engineering disasters such as slope instability, tunnel collapse, and dam failure. This issue is particularly significant in karst areas, where quantitatively analyzing the mechanisms underlying the deterioration of rock properties and changes in strength parameters under the influence of hydrochemical action is of great significance for assessing the long-term stability of geotechnical engineering. In this study, based on acid erosion tests of limestone and sandstone and focusing on the dissolution of calcareous cement, quantitative equations were derived using chemical kinetics theory and rock porosity variations. Based on the method that the dissolution process of cement can be equivalently represented as a reduction in the bonded contact area in PFC simulations, a new quantitative derivation process for the numerical simulation of microscopic damage in rock was proposed. By equivalently adjusting the particle bonded contact size and the coefficient of friction between particles, the loss of calcareous cement and the degradation of frictional properties on mineral particle surfaces were simulated respectively. With the bonded contact size serving as an intermediary, the interrelationship between chemical kinetics theory and porosity of rock was elucidated. The results show that the proposed method achieves high simulation accuracy, with relative errors of 1.59% and 3.78% for compressive strength and elastic modulus, respectively, in limestone simulations, and 8.0% and 5.66% in sandstone simulations. Under certain conditions, the porosity of calcareous cemented rock at any times during the acid–rock reaction can be quantitatively calculated using the derived theoretical formulas, providing a reference for the engineering community in assessing the long-term stability of rock mass.