Zero-dimensional (0D) inorganic perovskites have attracted great interest for white-light-emitting applications because of their broad band emissions originating from self-trapped excitons. In this work, we explore and decipher exciton self-trapping in a series of 0D inorganic perovskites, A4PbX6 and A4SnX6 (A = K, Rb, and Cs; X = Cl, Br, and I) at the density functional theory level within the theoretical framework of the one-dimensional configuration coordinate diagram. We demonstrate that the formation of self-trapped states in A4PbX6 and A4SnX6 can be attributed to local structural distortions of individual [PbX6]4– and [SnX6]4– octahedra. Importantly, with the goal of both potentially improving the stability of the Sn derivatives and enhancing the emission efficiency, we further propose and design two types of 0D perovskite heterostructures, bulk A4PbX6/A4SnX6 mixtures and A4PbX6/A4SnX6 heterojunctions. We find that these 0D heterostructures exhibit type-I energy level alignment in which energy transfer from A4PbX6 to A4SnX6 is strongly promoted. Interestingly, these heterostructures show an increase in the transition dipole moments between the ground and self-trapped states compared to the pristine 0D perovskites. Our findings provide a new material design strategy for boosting self-trapped emissions with improved air stability for white-light-emitting applications.