Abstract: Five kinds of hydroxycinnamic acid were synthesized by Knoevenagel condensation reaction, using hydroxy benzaldehyde derivatives, malonic acid, pyridine, piperazine hexahydrate.Structural identification and purity were confirmed by melting point analysis, _1H nuclear magnetic resonance (_1H NMR) , Infrared Spectroscopy (IR) , high performance liquid Chromatography (HPLC) .To evaluate its antioxidant activity, Fe³⁺ reducing power, DPPH free radicals scavenging activity were employed, and compared with hydroxy benzaldehyde derivatives, butylated hydroxytoluene (BHT) and cinnamic acid. Results showed that the products were identified as—3, 4-dihydroxycinnamic acid, 4-hydroxy-3-methoxycinnamic acid, 3-hydroxy-4-methoxycinnamic acid, 4-hydroxycinnamic acid, 3-hydroxycinnamic acid, whose purity were 92.45%, 93.14%, 99.55%, 96.54%, 93.50% respectively.3, 4-dihydroxycinnamic acid, 4-hydroxy-3-methoxycinnamic acid had strong reducing power on the Fe³⁺ and scavenging power on the DPPH radical, which were higher than that of BHT, and the IC₅₀ (half inhibitory concentration) of DPPH radical were (11.50 ± 0.02) , (16.57 ± 0.04) , (33.04 ± 0.03) μg/m L.Phenolic hydroxyl group was the antioxidant activity center of hydroxycinnamic acid derivatives, and hydroxycinnamic acid derivatives which had catechol structure or the hydroxyl's ortho-position with electron-donating group had strong antioxidant activity.4' hydroxy group on the benzene ring had strong antioxidant activity than 3' hydroxy group on the benzene ring. α, β-unsaturated acid conjugate structure connected with benzene ring could improved the antioxidant activity of phenolic compounds.