Health and safety of people are endangered by mass propagation of harmful algae in fresh waters such as lakes, reservoirs and rivers. The method for controlling algae bloom at present respectively has strong points, but has considerable boundedness, or limited effect or large cost, or has secondary pollution because of adding chemical substances, especially cannot solve the excessive multiplication of the harmful algae in a large range. The invention provides a method for controlling excessive multiplication of harmful algae in fresh water by a straw, and belongs to the technical field of water eutrophication treatment. The method is simple and effective and low in cost, and can easily restrain excessive multiplication of the harmful algae in the fresh water on a large scale. By adopting the method, a lot of agricultural wastes, namely straws are adopted as raw materials. Therefore, the method is wide in source, and convenient to carry out, and can be carried out in waters such as rivers, lakes and reservoirs on a large scale.