The effect of alkali solution reinforcing red clay was investigated in this paper in the aim of improving its mechanical strength. Temperature, dry density of red clay, and reinforcement method were set varied in the test. The mechanical properties of red clay treated with alkali under different stress paths were examined using a stress-strain controlled triaxial apparatus. Results demonstrated that the mechanical strength of alkali-treated red clay increased with the rising of curing temperature. At optimal temperature, the unconfined compressive strength increased by 90.2%; and given optimal dry density, the alkali-treated red clay sample witnessed a peak mechanical strength of 162.1 kPa. The reinforcement effect reached the optimum when alkali solution was blended into red clay. Moreover, triaxial shear test under different stress paths unveiled that the shear strength of alkali-treated red clay increased under the following three conditions: CTC (Conventional Triaxial Test which simulates the loading process with mean principal stress rising), RTC (Triaxial Test which simulates lateral unloading process with mean principal stress reducing), and TC (Triaxial Test which simulates lateral unloading and axial loading in the meantime with mean principal stress unchanged). The c value of red clay under effective stress path increased by 77.84% and 20.36% under CTC and RTC, respectively, and the value of φ changed little, whereas under the TC path, the c value remained basically unchanged, and the value of φ increased by 20.51%. The research results are of practical significance for the reinforcement of red clay foundation or foundation pit support design.