This study predicts the thermal conductivity of Mg–Al–La alloys using the CALPHAD method. Mg–Al alloys have high strength and ductility but low thermal conductivity due to Al addition. The study designed Mg–Al–La alloys with varying Al content and second phases (Al₂La, Al₃La, Al₁₁La₃) and solid solubility of Al in α-Mg. Experimental results showed that increasing Al content leads to a transformation of second phases from Al₂La to Al₃La to Al₁₁La₃. The negative effect of second phases on thermal diffusivity was Al₂La > Al₃La > Al₁₁La₃. Higher Al solubility in α-Mg reduced thermal conductivity. A database of reciprocal thermal diffusivity was established using the CALPHAD method, showing good agreement with experimental data (standard error ±1.2 W/(m·K)). The database can be used to design Mg–Al alloys with high thermal conductivity. La is an optimal alloying element due to its lower effect on reducing thermal conductivity compared to Ce. Mg–Al–La alloys have excellent mechanical properties, high-temperature stability, and good thermal conductivity. The La in the alloy forms Al₁RE₃ phases, which reduce Al content in α-Mg and improve thermal conductivity. However, the effects of solute atoms and second phases on thermal conductivity have not been thoroughly studied. The CALPHAD method allows prediction of thermal conductivity without extensive experiments. The study used CALPHAD to establish the relationship between reciprocal thermal diffusivity and temperature in the Mg–Al–La system. The influence of second phases and Al solubility on microstructure and thermal conductivity was studied. The thermal conductivity of Mg–Al–La alloys was predicted using the obtained database of thermal diffusion resistivity. The study used experimental samples prepared by melting and casting, with compositions analyzed by ICP and microstructure observed by SEM–BSE and EDS. Thermal diffusivity was measured using a Netzsch LFA467 device. The study provides a database for predicting thermal conductivity of Mg–Al–La alloys, which can guide the design of high thermal conductivity Mg–Al alloys.This study predicts the thermal conductivity of Mg–Al–La alloys using the CALPHAD method. Mg–Al alloys have high strength and ductility but low thermal conductivity due to Al addition. The study designed Mg–Al–La alloys with varying Al content and second phases (Al₂La, Al₃La, Al₁₁La₃) and solid solubility of Al in α-Mg. Experimental results showed that increasing Al content leads to a transformation of second phases from Al₂La to Al₃La to Al₁₁La₃. The negative effect of second phases on thermal diffusivity was Al₂La > Al₃La > Al₁₁La₃. Higher Al solubility in α-Mg reduced thermal conductivity. A database of reciprocal thermal diffusivity was established using the CALPHAD method, showing good agreement with experimental data (standard error ±1.2 W/(m·K)). The database can be used to design Mg–Al alloys with high thermal conductivity. La is an optimal alloying element due to its lower effect on reducing thermal conductivity compared to Ce. Mg–Al–La alloys have excellent mechanical properties, high-temperature stability, and good thermal conductivity. The La in the alloy forms Al₁RE₃ phases, which reduce Al content in α-Mg and improve thermal conductivity. However, the effects of solute atoms and second phases on thermal conductivity have not been thoroughly studied. The CALPHAD method allows prediction of thermal conductivity without extensive experiments. The study used CALPHAD to establish the relationship between reciprocal thermal diffusivity and temperature in the Mg–Al–La system. The influence of second phases and Al solubility on microstructure and thermal conductivity was studied. The thermal conductivity of Mg–Al–La alloys was predicted using the obtained database of thermal diffusion resistivity. The study used experimental samples prepared by melting and casting, with compositions analyzed by ICP and microstructure observed by SEM–BSE and EDS. Thermal diffusivity was measured using a Netzsch LFA467 device. The study provides a database for predicting thermal conductivity of Mg–Al–La alloys, which can guide the design of high thermal conductivity Mg–Al alloys.