Ultrathin pure bromide metal halide perovskite nanoplatelets (MHP NPLs) are expected to achieve high-purity pure blue and even deep blue emission. However, the optical properties of ultrathin NPLs are extremely unstable because they tend to self-assemble and fuse into large particles. Here, we find that the deep blue emitting NPLs can be stabilized by controlling the self-assembly behavior. By partial exchange of the surface ligand of aliphatic acid with aromatic acid, the self-assembly configuration can be effectively controlled, endowing stable face-to-face self-assembled configurations. The resultant NPL superlattices can retain blue emission for over half a month, while the control samples will red-shift within a few hours. The molecular simulation results reveal that the aromatic acid ligand has a strong passivation capacity on the NPL surface and moderate intermolecular van der Waals interactions. Therefore, by tuning the benzoic acid concentration, the intrinsic optical properties of individual NPLs can be stabilized in the form of self-assembly, avoiding crystal regrowth and exciton delocalization. This controllable self-assembly of ultrathin MHP NPLs benefits fundamental research in the field of perovskite nanocrystal superlattices and broadens the possibilities for perovskites for blue emission.