NOVELTY - Artificial in vitro preparation of amyloid peptide oligomer with low molecular weight, involves dissolving monomer of amyloid- beta polypeptide in anhydrous dimethyl sulfoxide, reacting the obtained amyloid- beta polypeptide with buffer system at 4 degrees C for 22-26 hours, and subjecting the amyloid- beta polypeptide to natural polymerization to obtain the oligomer.    USE - The method is useful for preparing amyloid peptide oligomer with low molecular weight artificially for preparing medicines and reagents for diagnosing, preventing and/or treating Alzheimer's disease, and as standard substance for preparing reagent for detecting Alzheimer's disease by enzyme-linked immunosorbent assay, Western blot experiment or transmission electron microscope (both claimed).    ADVANTAGE - The method is economic and environmentally-friendly, and obtains high stable amyloid peptide oligomer.    DETAILED DESCRIPTION - Artificial in vitro preparation of amyloid peptide oligomer with low molecular weight, involves dissolving monomer of amyloid- beta polypeptide using anhydrous dimethyl sulfoxide, reacting the obtained amyloid- beta polypeptide with buffer system at 4 degrees C for 22-26 hours, and subjecting the amyloid- beta polypeptide to natural polymerization to obtain an oligomer. The buffer system comprises (in mmol/l): sodium chloride (125), potassium chloride (3.3), potassium dihydrogen phosphate (1.2), sodium bicarbonate (26), calcium chloride (2.5), magnesium sulfate (2.4), glucose (5) and amino acid (0.5 g/l). INDEPENDENT CLAIMS are included for the following: (1) method for preparing the monomer of amyloid- beta polypeptide by gene recombination method, involves adding an amino acid label group (6XHis) at N-terminal, adding termination sub sequence (TGA) at C-terminal, constructing expression vector of amyloid- beta 42 monomer, expressing the vector in Escherichia coli expression system, purifying target polypeptide using nickel column, dissolving the monomer of amyloid- beta 42 in cold hexafluoroisopropanol under room temperature for 1-2 hours, volatilizing mixture and totally removing the hexafluoroisopropanol to obtain a peptide membrane; and (2) artificial cerebrospinal fluid comprising (in mmol/l): sodium chloride (125), potassium chloride (3.3), potassium dihydrogen phosphate (1.2), sodium bicarbonate (26), calcium chloride (2.5), magnesium sulfate (2.4), glucose (5) and amino acid (0.5 g/l).