(±)-7,8-Dihydroxy-3-methyl-isochromanone-4 [(±)-XJP] is a natural antihypertensive product contained in banana (Musa sapientum L.) peel. (-)-XJP and (+)-XJP were first obtained by chiral resolution, meanwhile circular dichroism (CD) calculations and chiral synthesis were employed to investigate the absolute configuration. The results indicated that the absolute configuration of (+)-XJP is S-configured and the absolute configuration of (-)-XJP is R-configured. Furthermore, the evaluation of antihypertensive effects in vivo proved that R-(-)-XJP was more potent than S-(+)-XJP and [(±)-XJP].