The invention belongs to the water processing field, especially relating to a method for preparing an Skeletonema costatuma inhibitor, a method for preparing Ulva linza Linnaeus extract for inhibiting Skeletonema costatuma: leaching Ulva linza Linnaeus with sterilized sea water, alcohol, acetone, and aether or chloroform, concentrating the obtained extract by rotary evaporation, putting the concentrated extract into a to-be-processed water body to inhibit red tide alga growth in the water body, where ordinarily, per kg Ulva linza Linnaeus is leached with 30-50 L sea water for 0.5-2 days, the obtained extract is concentrated to 10% of its original bulk, and 2-10 ml concentrate is putted into per L to-be- processed water body. And the invention fully uses Ulva linza Linnaeus in Ulva linza Linnaeus plants, effectively inhibiting growth of harmful marine red tide alga, especially Skeletonema costatuma, and can be widely applied to various water bodies to implement red tide.