Lead halide perovskite (LHP) nanocrystals (NCs) offer an easy tunability of band gap via anion exchanges, enabling their usage in tandem optoelectronic devices with graded band gaps. However, instantaneous anion migrations at the interfaces of different LHP layers impede the formation of well-defined interfaces. We deposited an ultrathin alumina (Al2O3-x) layer at the interface of CsPbBr3-CsPbI3 NC films by using atomic layer deposition (ALD) and demonstrated that low-temperature ALD alumina has negligible impact on the structural or optical properties of CsPbBr3 NCs except agglomeration. ALD alumina can restrict anion migration for months but cannot be prevented permanently. The rate of anion migration significantly decreases with an increase in the Al2O3-x layer thickness on CsPbBr3 NC films, which follows first-order kinetics. Density functional theory (DFT) calculations showed that the iodide ion can migrate through oxygen vacancies in the Al2O3-x layer with an activation energy of 1.54 eV. This strategy provides new insight into fabricating halide perovskite-based tandem optoelectronics devices.