The present invention relates to a composition for anti-green algae containing water-soluble free amine chitosan having the anti-algae activity against water-bloom forming algae as an active ingredient and a method for preparing the same. Particularly, the water-soluble free amine chitosan of the present invention can kill directly water-bloom forming algae, suggesting that it has excellent anti-green algae activity to inhibit water-bloom, and is non-toxic to freshwater organism cells. Therefore, the composition for anti-green algae containing the said water-soluble free amine chitosan as an active ingredient can be efficiently used as a safe anti-green algae composition applicable in the freshwater ecosystem including solidified dam, reservoir, lake, golf course water hazard, pond, freshwater fish farm, and fishing spot.
