Peat soils, characterized by high porosity, high natural moisture content, and high compressibility, often cause excessive settlement and stability issues in foundations. Traditional cement-stabilization methods suffer from limited improvement effects in organic-rich environments and high environmental costs. Taking the typical peaty soil around Dianchi Lake in Kunming as the research object, alkali-activated granulated ground blast-furnace slag (GGBS) combined with three types of short fibers (basalt fiber, polypropylene fiber, and polyacrylonitrile fiber) were innovatively employed to improve peat soils. Unconfined compressive strength tests, triaxial shear tests, and X-ray diffraction (XRD)/scanning electron microscope (SEM) microstructural analyses were systematically conducted. Results indicate that all three fibers significantly enhance the strength of the solidified soil, with polypropylene fiber yielding the best results due to its superior physical interlocking effect, after 28 days of curing, the unconfined compressive strength (UCS) reaches 433.9 kPa, representing a 61.1% increase compared to the fiber-free sample. Fiber reinforcement markedly improves specimen ductility. Polypropylene fiber-modified soil exhibits the highest cohesion (140.36 kPa) in triaxial tests, with substantially enhanced shear strength. Microscopic analysis reveals interlaced fiber distribution within the matrix, forming a fiber skeleton that suppresses particle displacement, with different interfacial bonding mechanisms observed for different fibers. The structural uniform hardening (SUH) model is applied to this type of modified soil for the first time, accurately simulating the triaxial stress-strain curves and reproducing the full-range strain hardening response with correlation coefficients R² exceeding 0.98. The research results provide a theoretical basis and technical support for the green and efficient reinforcement of high-organic-content soft soils in this region.