The present invention belongs to the field of drug synthesis, and relates to a method for preparing a substituted phenylacetic acid derivative, specifically a method for preparing 2-[4-(2-oxopentylmethyl)phenylpropionic acid]. A precursor compound (I) of loxoprofen is prepared by reacting a dihalobenzyl compound or a disubstituted benzyl compound with a cyclopentanone group or a precursor compound thereof, wherein X is a halogen, L1 is a halogen, OH, OMs, OTs or OTf, L2 is a halogen, a cyano, a hydroxyl, -CH2OH, -CHO, nitromethane, an ester group, -NR4R5, OTf, OTs, OMs, -C=CR6 or -C≡CR7, R4, R5, R6 and R7 are a low substituted alkyl, Z is a cyclopentanone group or a precursor form thereof, the precursor form being (II), (III), (IV), (V) or (VI), and R3 is a low substituted alkyl. Further, the method comprises a step of converting the precursor form of the cyclopentanone group to the cyclopentanone group. The precursor compound of loxoprofen is used for preparing loxoprofen compounds.