<p>Alkali-contaminated soil is a severe global issue. Red clay, widely dispersed throughout southern China, is commonly employed as roadbed and foundation soils. As a result, it is vital to investigate the changes in shear strength and stress–strain relationships of alkali-contaminated soils. To examine the impact of alkali contamination on red clay's shear strength and stress–strain behavior, we conducted indoor consolidated undrained triaxial shear experiments on red clay samples contaminated with various pH levels (pH 7, pH 9, pH 10, pH 11). The findings indicated that the cementation effect of free oxides in red clay was compromised, and the interparticle connection structure was disrupted due to alkali contamination. As the pH increased, the shear strength of red clay exhibited a persistent decline. Next, an investigation was conducted into the stress–strain normalized characteristics of alkali-contaminated red clay, utilizing the stress–strain relationship curves acquired from the conducted experiments. Ultimately, a prediction equation for the stress–strain relationship of red clay under varying pH pollution was established by matching the stress–strain parameters and utilizing the asymptotic value of the principal stress difference as the normalization factor. The prediction equation accurately forecasts the stress–strain correlation of red clay under varying pH contamination. This study has enhanced comprehension of the shear strength and normalized characteristics of red clay polluted with alkali.</p>