the preparation method is to put 3-methylbutene [2] alcohol and triethyl orthoacetate into the reaction pot, then add phenol as catalyst, stir and gradually heat to 140 ℃, stir while heating and stirring, evaporate ethanol, react for about 20 hours, evaporate without ethanol, move to the distillation pot for decompression distillation, remove triethyl orthoacetate first, and then collect 74~78 ℃/7.33kPa fraction, which is 3, ethyl 3-dimethylpentene [4] acid has a yield of about 75%. In this reaction, triethyl orthoacetate should be excessive to avoid the formation of isopentenyl benzoate. In addition to phenol, the catalyst can also be phosphoric acid, isobutyric acid and hydroquinone.