Methods for preparing a recombinant protein from a bacterium are provided. The method includes constructing an expression vector including two promoters. Each of the two promoters attaches a secretion signal peptide to one polypeptide of a protein. The protein attached with the two promoters and secretion signal peptides is then cloned into the expression vector to provide a recombinant expression plasmid. The recombinant expression plasmid is transformed into a host cell. A fermentation process is performed to grow the host cell and to induce an expression to synthesize polypeptides in the host cell and to transport the polypeptides to an outside of a cytoplasm of the host cell, such that the polypeptides are released in a soluble form in a growth medium of the host cell. The polypeptides are assembled into a three-dimensional structure of the protein. The protein is captured from the growth medium.