The invention discloses a preprocessing method for the pure culture of a microcystis algal strain. Before a streak plate method is implemented, algal suspension is subjected to constant-temperature water bath heating under an alkalescence condition to remove attached bacteria in microcystis extracellular polysaccharides so as to efficiently obtain the sterile microcystis algal strain. Under an alkaline condition, the dissolving of EPS (Expandable Polystyrene) can be accelerated by warm water bath heating, and the attached bacteria attached to the polysaccharide can be more effectively removed; washing shock vibration and water bath heating can be combined to more efficiently remove the attached bacteria, so that the streak plate method can more efficiently carry out a separation and purification process, and streaking frequencies are reduced.