Formability and stability issues are of core importance and difficulty in current research and applications of perovskites. Nevertheless, over the past century, determination of the formability and stability of perovskites has relied on semi empirical models derived from physics intuition, such as the commonly used Goldschmidt tolerance factor, t. Here, through high-throughput density functional theory (DFT) calculations, a database containing the decomposition energies, considered to be closely related to the thermodynamic stability of 354 halide perovskite candidates, is established. To map the underlying relationship between the structure and chemistry features and the decomposition energies, a well-functioned machine learning (ML) model is trained over this theory-based database and further validated by experimental observations of perovskite formability (F 1 score, 95.9%) of 246 A 2 B(I)B(III)X 6 compounds that are not present in the training database; the model performs a lot better than empirical descriptors such as tolerance factor t (F 1 score, 77.5%). This work demonstrates that the experimental engineering of stable perovskites by ML could solely rely on training data derived from high-throughput DFT computing, which is much more economical and efficient than experimental attempts at materials synthesis. demonstrate enhanced stability in comparison to their single perovskite counterparts. For example, although the photoactive perovskite phases of RbPbI 3 , CsPbI 3 , and FAPbI 3 are unstable at room temperature, the proper mixing of (Rb, Cs, and FA) on the A site can result in a stable mixed-A-site perovskite. [21,23] Currently, for the precise experimental control of stability, more mixing elements, such as triple-A (Cs, MA, FA)InBiBr 6 , [26] triple-A double-X Cs x (MA 0.17 FA 0.83 ) (1-x) Pb(I 0.83 Br 0.17 ) 3 , [20] and quadruple-A double-X Rb-FA 0.75 MA 0.15 Cs 0.1 PbI 2 Br, [27] are used, making the problem more complicated. Further insight is required to understand the effect of elemental mixing and provide guidance for stability engineering, especially relating to the type of mixing elements and their concentrations.