The structural and mechanical characteristics of carbonate rock under the action of chemistry and mechanics is an important research topic for the evaluation of the effectiveness of acid fracturing technology in this kind of reservoir. This research focused on the impact of 20% HCI gelled acid on the structural and mechanical properties of carbonate rocks, categorized into four types based on their mineral composition: limestone, dolomite-bearing limestone, limestone-bearing dolomite, and dolomite. The experiments revealed distinct reactions of these rock types to acid exposure. Limestone exhibited uniform etching, while dolomite-bearing limestone showed selective etching, creating wormhole-like grooves. Limestone-bearing dolomite and dolomite predominantly experienced point etching and erosion along structural planes. Initially, the shear failure of carbonate rocks was primarily governed by matrix strength. However, acid treatment altered their internal structure, making them more susceptible to tensile stress damage, leading to potential splitting or destruction along structural planes. Notably, the reduction in the macroscopic strength of the carbonate rocks post-acid treatment was significantly greater than the decrease in matrix strength alone. The invasion of acid liquid into the rocks introduced additional microscopic defects, evidenced by a reduced proportion of elastic energy and an increased proportion of dissipated energy at peak stress levels. This suggests that the macroscopic mechanical property deterioration results from both matrix strength weakening and internal structural changes. These findings offer valuable insights for field acid fracturing operations in carbonate rock reservoirs and aid in the planning of subsequent production strategies