The invention discloses a method for preparing an intermediate 3-methyl-2-butenoic aldehyde. The prior method uses a solvent with strong toxicity, and has rigorous reaction conditions, large separation difficulty and high cost. The method has the following steps: putting 2-methyl-3-butine-2-hydrin and a catalyst into a reaction kettle; performing a catalytical recomposition reaction through raising the temperature to obtain a mixture of the 2-methyl-3-butine-2-hydrin and the 3-methyl-2-butenoic aldehyde; separating the vaporized 2-methyl-3-butine-2-hydrin and the 3-methyl-2-butenoic aldehyde in the reaction process through a rectification tower; recovering the 2-methyl-3-butine-2-hydrin and putting the 2-methyl-3-butine-2-hydrin back into the reaction kettle for feeding; discharging materials from one side of the rectification tower to obtain the 3-methyl-2-butenoic aldehyde crude product; and replenishing new 2-methyl-3-butine-2-hydrin into the reaction kettle continuously for reaction. A solvent is not used in the method, the reaction temperature is gentle and easy to control, and the continuous production is realized by continuous feeding and discharging.