Based on the triaxial test data of three different working conditions of CTC (increasing p), TC (equivalent p), and RTC (decreasing p) before and after alkali treatment and combined with Duncan-Chang model and generalized strength, the theory (Extend SMP) studies the stress-strain relationship of soil samples. The results show that based on the Duncan-Chang model, the ε₁/(σ₁-σ₃)~ε₁ relationship curve of red clay samples before and after alkali treatment is not a simple linear relationship. The relationship between stress and strain under the simple Duncan-Chang model may not apply. Based on the generalized strength surface theory, the stress ratio and stress increment ratio of the sample are simple linear relationships. The stress ratio and stress ratio are a curve relationship between the front and back sections being gentle and the middle being steep, The relationship between stress ratio and axial strain has high similarity with the change of strain curve. The rapid increase first and then the slow increase curve finally stabilize, and the stress ratio of red clay on the Extend SMP surface after alkali treatment has a significant peak.