Abstract: To study the mechanical properties of deep frozen peaty soil in the lacustrine deposits of Dianchi Lake, the experimental method of "long-term high-pressure K₀ consolidation—loaded freezing—loading" is adopted. The effects of freezing temperature, confining pressure and water content on the mechanical properties of frozen peaty soil are explored through indoor low-temperature triaxial experiments, and the theory of binary medium constitutive model is introduced to analyze the deviatoric stress-strain relationship. The results show that the coefficient of earth pressure at rest (K₀) value of peaty soil is 0.5. The deviatoric stress-strain curve of frozen peaty soil can be divided into the linear elastic growth stage, the elastoplastic deformation stage and the stable stage. The peak strength is positively correlated with the confining pressure and water content, and negatively correlated with temperature. The cohesion and internal friction angle fluctuate within the ranges of 418.72~571.96 kPa and 14.53°~17.72° respectively. With the decrease of temperature, the cohesion increases and the internal friction angle decreases. Based on the experimental results, a binary medium constitutive model suitable for deep frozen peaty soil is constructed, and its rationality is verified, demonstrating high applicability and accuracy.