Abstract: The improved soil has a wide application potential in foundation treatment, pipeline protection, energy absorption and vibration reduction in the field of energy rock and soil. The researches on the correlation between thermal and mechanical properties of soil closely related to the energy rock and soil are of important reference value for guiding the design, construction and maintenance of underground energy structures. To reveal the evolution laws of the thermal and mechanical properties of silt improved by recycled polyester fiber and inorganic curing agent with curing age, the macroscopic tests (compaction, unconfined compressive strength, resilience modulus), microscopic tests (mercury injection and scanning electron microscopy) and thermal conductivity tests are carried out. The changes of thermal properties (thermal conductivity), mechanical properties (strength, resilience modulus), content of recycled polyester fiber, water content and age of the improved silt are analyzed. The changes of pore size and microstructure of the improved silt are also discussed. The internal relationship between thermal and mechanical properties of the improved silt is expounded. The results show that the addition of the fiber increases the optimum moisture content of silt. The thermal conductivity of the improved silt decreases with the increase of the fiber content and age. After 28 days of age, the change of water content has few effects on the thermal conductivity of the improved silt. The road performance of improved silt meets the design requirements of highway subgrade. The cementing materials derived from the improved silt can wrap particles and fill pores, and inhibit the internal heat conduction of the silt. The thermal conductivity of the improved silt decreases with the increase of the compressive strength and resilience modulus, showing a negative correlation.