Red clay, characterized by high moisture content, high plasticity, large porosity, high strength, and low compressibility, is widely used in engineering projects. The engineering properties and deformation modeling of red clay in Guiyang differ significantly from those of ordinary soils. Consequently, it is essential to conduct a detailed study of the stress-strain relationship of red clay in order to obtain accurate engineering parameters. Hence, a series of unconsolidated undrained triaxial compression tests (UU) were conducted on red clay in Guiyang under different water contents and confining pressures. The test results show that the stress-strain curve of the clay undergoes three stages: elastic deformation, plastic yielding, and stress reduction after failure. After reaching peak deviatoric stress, the deviatoric stress decreases gradually and stabilizes, showing noticeable strain-softening characteristics. The nonlinear model from Nanjing Hydraulic Research Institute (NHRI) was introduced for fitting and obtaining relevant parameters. Then, based on the FLAC 3D platform, numerical simulation was carried out on a standard soil sample. The research results show that the calculated curve from the nonlinear model fits the experimental curve quite well, and the numerical test curve also agrees with it. This indicates that the nonlinear model from NHRI can effectively reflect the strain-softening behavior of clay and is highly applicable to Guiyang red clay.