Adding 500g of phosphoric acid catalyst into a 2000ml three-neck flask, adding 150ml of methanol under stirring, heating to 55 ℃, slowly adding 50g of piperonyl, dropping after 0.5 hour, adding 82g (0.59mol) of urotropine, starting to heat to 80 ℃, preserving heat for 7 hours, sampling, carrying out liquid phase analysis, cooling to 60-70 ℃, adding 700ml of water, stirring for 3 hours, adding ethyl acetate for two-time extraction, combining organic phases, adding saturated salt solution for washing, separating out the organic phase, adding 40g of anhydrous sodium sulfate for drying, filtering, concentrating ethyl acetate at 40 ℃ of filtrate, and carrying out reduced pressure distillation on residual oil pump to obtain 31g of piperonal with the purity of 9