alpha-sheet conformation has been proposed to exist as an intermediate conformational component and mediate the fibrillar assemblies of amyloid proteins at certain conditions. However, less attention has been paid to this form of conformation in the studies of the mechanism of amyloidosis because there is insufficient evidence for the existence of the alpha-sheet intermediate and the transition from the alpha-sheet intermediate to the beta-sheet fibril. Herein, we characterized the structures of a D,L-alternating amyloidogenic decapeptide under different conditions and studied the structural conversion between the alpha-sheet and beta-sheet in the fibrillation processes of the peptide. We obtained for the first time the image of alpha-sheet structured fibrils in aqueous solution and found the essential role of water molecules in the stabilization of the alpha-sheet structure. We also provided experimental evidence of the structural conversion from the alpha-sheet to the beta-sheet in the peptide aggregates.