This study investigates the thermal conductivity of Mg–Al–La alloys, which have excellent strength and ductility but relatively low thermal conductivity due to the addition of aluminum. The research aims to establish a database for predicting temperature- and composition-dependent thermal conductivities. The study designed Mg–Al–La alloys with varying contents of Al1La, Al1La, and Al11La3 phases and solid solubility of Al in the α-Mg phase. Experimental results showed that the second phase transformation from Al-La to Al1La and further to Al11La3 occurs with increasing Al content at a constant La amount. The negative effect on thermal diffusivity was highest for Al1La, followed by Al1La, and then Al11La3. Increasing the solid solubility of Al in α-Mg significantly reduced thermal conductivity. A database of reciprocal thermal diffusivity for the Mg–Al–La system was established using the CALPHAD method, with a standard error of ±1.2 W/(m·K). The predicted results were in good agreement with experimental data, providing valuable guidance for designing Mg–Al alloys with high thermal conductivity.This study investigates the thermal conductivity of Mg–Al–La alloys, which have excellent strength and ductility but relatively low thermal conductivity due to the addition of aluminum. The research aims to establish a database for predicting temperature- and composition-dependent thermal conductivities. The study designed Mg–Al–La alloys with varying contents of Al1La, Al1La, and Al11La3 phases and solid solubility of Al in the α-Mg phase. Experimental results showed that the second phase transformation from Al-La to Al1La and further to Al11La3 occurs with increasing Al content at a constant La amount. The negative effect on thermal diffusivity was highest for Al1La, followed by Al1La, and then Al11La3. Increasing the solid solubility of Al in α-Mg significantly reduced thermal conductivity. A database of reciprocal thermal diffusivity for the Mg–Al–La system was established using the CALPHAD method, with a standard error of ±1.2 W/(m·K). The predicted results were in good agreement with experimental data, providing valuable guidance for designing Mg–Al alloys with high thermal conductivity.