[Objective] The effect of simulated acid rain on the shear strength characteristics of red clay was investigated and the influence strength mechanism and microstructure of red clay after acid rain corrosion was analyzed in order to provide theoretical and engineering guidance for the construction of infrastructure in acid rain area, especially the protection of roadbed slope and safe operation of infrastructure. [Methods] The red clay of a roadbed slope in Kunming City, Yunnan Province was used as the research object. The study adopted the static soaking test and used pure water and acid liquor with different concentrations to soak and erode the red clay for 7, 14, and 28 days. Thereafter, the shear strength of the red clay was measured using the direct shear tests, and the microstructures of the eroded soil samples were analyzed via electron microscope scanning and energy spectrum analysis to analyze the influence mechanism of strength. [Results] ① The effect of acid liquor on the stress-strain characteristics of red clay is significant, especially under high concentration acid. The stress-strain curve was unstable and increased like a step, with strain hardening characteristics. ② With the increase in soaking time and acid concentration, the shear strength of red clay and its index of cohesion and internal friction angle gradually decreased, and the acid concentration had a more significant effect on the strength and its index. ③ Acid liquor decreased the sequioxide and the interparticle adhesion and bonding force and simultaneously increased the internal pores, loosened the structure, and increased the soil moisture content, thereby resulting in a decrease in shear strength and cohesion; however, the overall decrease in particle size caused by acid was the main reason for the decrease in the internal friction angle. [Conclusion] Acid rain has a significant effect on the strength of red clay from roadbed slope, and the intensity of acid rain has a more significant effect on the strength than the duration of rainfall. Attention should be paid to the prediction and prevention of roadbed slope stability and the safe operation of highway infrastructure.