On the basis of the unique chaotropic supramolecular assembly of cucurbit[5]­uril (CB5) and dodecahydro-closo-dodecaborate anion [closo-B12H12]2–, we have developed an efficient and universal platform to fabricate shape-controlled dodecaborate-based supramolecular organic frameworks (BOFs) decorated with ultrafine monodispersed trimetallic alloys. Simply by regulating the molar ratio of CB5 and [closo-B12H12]2–, a series of fascinating morphologies, such as flowerlike structures, nanorods, nanocubes, and nanosheets, were successfully constructed. These obtained BOFs were proved to be good substrate supports for in situ synthesis of trimetallic PtCoNi nanoalloys, where the final PtCoNi–BOFs materials were obtained efficiently as a precipitate from aqueous solutions, and showed excellent catalytic performance in ammonia borane hydrolysis with a high turnover frequency of 1490 molH2 molPt –1 min–1 and a low activation energy of 15.79 kJ mol–1.