Disclosed herein is a method for preparing a coumarin compound of formula (F), in which R1, R2, and R3 are independently H, C1˜C7 alkoxy, C1˜C7 alkyl, phenoxy, benzyloxy, or a halogen atom; R4 is an alkyl group; and Ar is an optionally substituted aryl group, the method including: treating a chromene compound having the following formula (E) with an acid in the presence of water. A chromene compound of formula (E) and a method for preparing the chromene compound of formula (E) are also disclosed.