A preparation method of sub-nano palladium alumina for double-bond isomerization comprises the steps of I, using chloro-palladic acid to synthesize palladium nano solution via a solution process; II,mixing the palladium nano solution of step I and an organic amine solution having molar concentration of 0.01-10 mol/L, and allowing reacting for 1-3 h, wherein the molar ratio of palladium to organicamine is 0.05-10; III, after the step II, centrifuging the solution to obtain sub-nano palladium cluster molecular solution; IV, weighing alumina powder of certain mass, subjecting the sub-nano palladium cluster molecular solution of step III according to the loading capacity of 0.5% by mass to over-impregnating for 2-12 h, filtering, washing, and drying to obtain sub-nano Pd/Al2O3 catalyst. Thepreparation method has the advantages that, for instance, catalytic activity is high, target product selectivity is good, the catalyst is reusable, and circulation stability is good.