Abstract: In this work, the early hydration reaction and mechanical evolution characteristics of graded fine tailing cemented backfill are studied. The hydration exothermicity and electrical resistance characteristics of backfill slurry with different lime sand ratios are tested, and the microscopic morphology characteristics of early hydration products are analyzed according to scanning electron microscopy (SEM). Finally, on the basis of uniaxial compression mechanical test results, the early hydration reaction process and the effect of the products on the strength evolution of the backfill are analyzed. The results showed that the exothermic process of slurry hydration underwent rapid reaction stage I, induction stage II, acceleration stage III, deceleration stage IV, and stabilization stage V. The volume resistivity underwent increasing stage I, decreasing stage II, and accelerated increasing stages III. The slurry lime sand ratio affects the hydration heat release and volume resistivity. The larger ratio is, the greater the hydration heat release rate and heat release, the more hydration products are generated, and the greater the volume resistivity. An increase in the lime sand ratio prolongs the induction time of the hydration reaction and increases the rate of the hydration reaction during the induction period. At the same time, it retards the growth of volume resistivity. The larger the lime sand ratio is, the stronger the retarding effect and the larger the retarding effect on the growth of slurry volume resistivity. The rate of the hydration reaction directly determines the formation speed of the backfill strength. When the main components of the filling material, C₃S and C₃A, are dissolved rapidly in water, much heat is released. The generated Ca(OH)₂ reduces the volume resistivity of the slurry and accelerates the growth of the backfill strength; the AFt generated subsequently compacts the pores between particles, blocks the dissolution of ions, decreases the rate of heat release, and prevents the growth of backfill strength. The backfill strength increases rapidly from 0–3 d and slowly from 3–7 d. When the hydration reaction is basically completed after 14 d, the filling material is solidified overall, and its strength is basically stable. The change in backfill strength first increases and then gradually decreases until stabilizing. These research conclusions provide theoretical support and scientific guidance for mining to adopt graded fine tailings to fill the underground goaf and control the temperature of the deep stope.