The invention belongs to the technical field of biotechnology and gene engineering, and particularly relates to an A[beta]42 modified protein, and an expression and purification method of the A[beta]42 modified protein in escherichia coli. An A[beta]42 expression system constructed by the method provided by the invention provides a construction method of a human amyloid protein A[beta]42 expression vector of prokaryotic expression; the expression vector is introduced into an escherichia coli expression host, and a targeted A[beta]42-His protein with high-purity biological activity is obtainedafter induced expression, two-step purification and identification. A large intestine expression system adopted by the method has the characteristics of high expression efficiency, large expression quantity, low cost, easy operation and the like. Obtained A[beta]42 polypeptide contains an His tag and is convenient to purify; the purity is high; and the yield is high. Compared with chemical synthesis of the A[beta]42, the method provided by the invention has the advantages of better aggregation characteristic, cytotoxicity and the like.