The invention discloses an allelochemical algistat and an extraction and purification method thereof. The allelochemical algistat is dioctyl adipate, and can be prepared by extracting cladophora serving as large marine algae by organic solvents and then carrying out chromatographic purification by silica gel-alumina; the content of dioctyl adipate exceeds 98.6%. Compared with the prior art, the allelochemical algistat, namely dioctyl adipate, is capable of effectively inhibiting the growth of red-tide algae comprising heterosigma akashiwo and gymnodinium brevis and shows remarkable concentration-dependent effect; the EC50-96h value can be 5.82 mg/L and 2.74 mg/L respectively. The allelochemical algistat is prepared from natural plant materials and high in environmental affinity, excellent in algal inhibition effect, free of limit of region and climate, simple in extraction and purification process, and can be widely applied to control and inhibition of red-tide algae in eutrophication water in offshore areas and aquaculture ponds.