The lattice thermal conductivity of minerals plays a critical role in controlling heat flow and temperature distribution in the Earth’s interior. Calcite, primarily composed of calcium carbonate (CaCO₃), can be subducted into the deep Earth and serves as an important carbon source. As pressure and temperature conditions change with depth, CaCO₃ undergoes phase transitions and thermal decomposition, which significantly affect its physical properties. In this study, we investigate the effects on thermal conductivity of calcite induced phase transitions and thermal decomposition using first-principles calculations combined with lattice dynamics. Our results show that the calcite Ⅰ → calcite Ⅱ phase transition leads to a reduction in thermal conductivity, whereas subsequent phase transitions at higher pressures result in increase. The thermal conductivity of aragonite and post-aragonite increases nearly linearly with pressure increasing, and the latter exhibiting a stronger pressure dependence. Upon thermal decomposition, the CaO exhibits significantly higher thermal conductivity than that of calcite, which may enhance local heat transfer. Analysis of relevant thermodynamic parameters indicates that the changes in thermal conductivity induced by phase transitions and decomposition are collectively determined by phonon group velocity and anharmonic scattering rates.