Mandelic acid was oxidized into vanillin by utilizing a soluble Cu(II) complex. When the molar ratio of CuSO4.5H2O to mandelic acid was 2.5, pH value was 13.5, reaction temperature was 95℃ and reaction time was 8 h, the oxidation yield reached 95.6%. The copper (II) salt can be efficiently recovered in H2SO4 solution by the oxidation of Cu2O in the presence of H2O2, and the activity of the regenerated copper (II) salt has no significant decrease. The yield of vanillin had almost no change each time, and kept at about 95.4% after ten cycles.