A method and an apparatus for hydrolyzing an organic solid are described. The method includes mixing an organic solid and a nanobubble water having a plurality of nanobubbles, to form an organic liquid, in which the nanobubbles contain a combustible gas; and applying an ultrasonic wave on the organic liquid, such that the nanobubbles generate an additional cavitation effect. A preprocessor is applicable to an organic solid processing system having an anaerobic digestion tank, in which the anaerobic digestion tank has anaerobic microbes for generating a combustible gas. The preprocessor includes a nanobubble water generator, a digestion tank, and an ultrasonic wave generator. With the method and the structure, the nanobubbles are used to increase the probability of generation of the additional cavitation effect, and the combustible gas is used to improve an impact force of bursts produced by the cavitation effect.