The missense mutation S20G in the human islet amyloid polypeptide (hIAPP) is supposed to be associated with the early onset of type II diabetes (T2DM) in Asian population. How such a single-point mutation induces variation of the conformation distribution and the amyloidogenic properties of hIAPP is still unknown. We studied the conformational ensembles of hIAPP segment 15-27 in both wild type and the S20G mutant type by performing extensive replica exchange molecular dynamics simulations in explicit solvent model. Our results reveal that the residue G20 initiates a turnlike structure in the neighborhood, facilitating the formation of long-ranged contacts. Moreover, we find that the point mutation favors protofibril model of full length hIAPP suggested recently based on experimental measurements. Such a conformational preorganization will decrease the entropy cost during the process of peptide self-assembly, which is a possible explanation of the faster fibrillation of hIAPP S20G mutant than that of the wild type found by experiments.