Abstract: ω-Isothiocyanomethyl longifolene(Ib) with low irritating odor was selectively synthesized by substitution reaction of lead thiocyanate and ω-chloromethyl longifolene(Ia) under reflux heating with hexane as solvent. The final product was characterized through HRMS, FT-IR, UV, ¹H NMR and ¹³C NMR. The molecular formula of C₁₇H₂₅NS was determined by HRMS, and the presence of N=C=S structure and long-leaf alkene structure was confirmed by FT-IR, UV spectroscopy and ¹H NMR and ¹³C NMR spectroscopy, respectively. Compound Ib could be regarded as an allyl isothiocyanate compound, containing longifolene and methyl isothiocyanate molecule structure moiety. The algae inhibitory activity of compound Ib was evaluated by 96-well plate two-fold dilution assay. It was obtained that compound Ib was able to inhibit the growth of Scenedesmus obliquus and Skeletonema sp. with the MIC of about 62.5 and 31.2 mg/L, respectively, and the algae inhibitory activity was significantly better than that of compound Ia, longleafene and methyl isothiocyanate.