The invention provides an expression method of phospholipase A1 accessory protein PlaS. The expression method comprises the following steps: removing signal peptide from the end N of a phospholipase A1 accessory protein PlaS gene to obtain truncated accessory protein, then constructing an escherichia coli/truncated accessory protein expression strain, and inoculating the expression strain into a culture medium to perform target protein inducible expression. The inventor adopts an end-N truncating technology to truncate the end N of the plaS gene by 35AA, and successfully constructs the BL21/dSP28 expression strain, so that a large amount of the accessory protein can be expressed. Furthermore, through optimization of components for expressing a culture medium, the culture medium which is capable of realizing soluble expression of a large amount of target protein and is easy to purify is obtained. The target protein is purified by adopting different purification modes, and an optimal purification method is determined. The culture medium enables a large amount of the target protein to be solubly expressed in broken supernatant, so that tedious operation of denaturization and renaturation in a protein purification process is reduced; the purification method increases the protein yield, so that the purification process is simpler and more convenient.