Understanding the early-age hydration kinetics of cemented paste backfill (CPB) is necessary for its successful application. In this paper, the early-age hydration heat evolution of CPB was experimentally studied using an isothermal calorimeter considering the coupled effects of cement content and temperature. We employed the Krstulovic-Dabic kinetic model to investigate the hydration mechanism and kinetic parameters of CPB. The results showed that increasing the tailings-cement ratio (TCR) decreased the hydration heat flow (N) and cumulative heat (Q) of CPB due to the decreased amount of cement. Additionally, increasing the TCR elongated the induction stage while shortening the acceleration stage. Increasing the temperature, increased the peak value of N, shortened its appearance time, and shortened/diminished the induction stage due to the increased chemical kinetics of hydration. The exponent and rate constant values in the Krstulovic-Dabic model decreased with the increase of TCR and the decrease of temperature. The hydration mechanism was nucleation and crystal growth (NG) → phase boundary reaction (I) → diffusion (D) for most CPB slurries, except for CPB slurries at TCR = 10/12 and temperature = 40 °C, where the hydration mechanism was changed to NG → D. This study indicates that the isothermal calorimeter, together with kinetic models, is a reliable tool for investigating the hydration kinetics of CPB, which can be incorporated into CPB design in the future.