To study the chemical constituents from Cuscutae Semen.The compounds were isolated and purified by various modern chromatographic techniques,including column chromatography (Sephadex LH-20 or silica gel) and preparative HPLC,and their structures were identified by spectroscopic data and physicochemical properties.Six compounds were isolated from the MeOH extract of Cuscutae Semen,which were elucidated as cuscutalin A (1),hyperoside (2),quercetin 3-O-β-D-apiofuranosyl-(1 →2)-β-D-galactoside (3),quercetin (4),kaempferol (5),and sesamin (6).Compound 1 is a new β-carboline alkaloid.Compounds 2 and 5 exhibited strong inhibitory activity against lysine specific demethylase 1 with the IC₅₀ values of 5.74±0.49 and 3.94±0.27 μmol/L,respectively.