In this work, the hypoglycemic components in Platycladi Cacumen, an essential traditional Chinese medicine, were evaluated by combining phytochemical investigation, spectrum-effect relationship analysis, and chemometric methods. The phytochemical studies on Platycladi Cacumen extract lead to the isolation of 21 potential bioactive compounds. The chromatographic fingerprints of Platycladi Cacumen samples were established by high-performance liquid chromatography. The hypoglycemic effects of Platycladi Cacumen samples were further evaluated by inhibition of α-glucosidase and detected by the high-performance liquid chromatography method. The spectrum-effect relationship study by bivariate correlations analysis and orthogonal partial least squares regression revealed that myricitrin (P9), quercitrin (P13), afzelin (P18), and amentoflavone (P24) were more relevant to the α-glucosidase inhibitory activity. The results of α-glucosidase inhibitory activity of 21 isolated compounds and molecular docking studies also indicated these flavonoids had potent α-glucosidase inhibitory activity. Collectively, the present study established the spectrum-effect relationship mode of Platycladi Cacumen and discovered the major hypoglycemic components, which provides a feasible method for screening bioactive components.