Zero-dimensional (0D) inorganic perovskites have attracted great interest for white-light-emitting applications due to their broadband 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 distor-tions 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 heter-ostructures, 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.