The invention relates to a solid-liquid separation method for oxidizing solution in synthesis of vanillin. The method performs solid-liquid separation by multistage separation, and comprises that: the oxidizing solution in the synthesis of the vanillin enters a sedimentation centrifuge for centrifugal separation, and a liquid phase and a solid phase are obtained; the liquid phase enters a flottweg separator for centrifugal separation, and a light phase and a heavy phase are obtained; and the light phase enters a precision filter for filtering separation, filter liquor and a filter cake are obtained, the filter liquor is clear and transparent, and good solid-liquid separation is achieved. Compared with the prior art, the method has reasonable process and simple operation, selects the most proper solid-liquid separation devices according to the characteristics of the oxidizing solution in the synthesis of the vanillin and the characteristics of the devices, realizes complete solid-liquid separation in the oxidizing solution, and separates and recycles a solid fine particle oxidant and reduction products thereof from the oxidizing solution so as to control the loss rate of the oxidant in a recycle system below 1 percent and greatly reduce the production cost of the vanillin.