Abstract: Mg−Al alloys have excellent strength and ductility but relatively low thermal conductivity due to Al addition. The accurate prediction of thermal conductivity is a prerequisite for designing Mg−Al alloys with high thermal conductivity. Thus, databases for predicting temperature- and composition-dependent thermal conductivities must be established. In this study, Mg−Al−La alloys with different contents of Al₂La, Al₃La, and Al₁₁La₃ phases and solid solubility of Al in the α-Mg phase were designed. The influence of the second phase(s) and Al solid solubility on thermal conductivity was investigated. Experimental results revealed a second phase transformation from Al₂La to Al₃La and further to Al₁₁La₃ with the increasing Al content at a constant La amount. The degree of the negative effect of the second phase(s) on thermal diffusivity followed the sequence of Al₂La > Al₃La > Al₁₁La₃. Compared with the second phase, an increase in the solid solubility of Al in α-Mg remarkably reduced the thermal conductivity. On the basis of the experimental data, a database of the reciprocal thermal diffusivity of the Mg−Al−La system was established by calculation of the phase diagram (CALPHAD) method. With a standard error of ±1.2 W/(m·K), the predicted results were in good agreement with the experimental data. The established database can be used to design Mg−Al alloys with high thermal conductivity and provide valuable guidance for expanding their application prospects.