The invention discloses a preparation method and application of a monoamine oxidase A inhibitor. The preparation method comprises the steps: S1, Chinese prickly ash and an ethanol solution are placedin to a container, and an extract solution is obtained through heating reflux; S2, ethanol is removed after reduced-pressure concentration, and an aqueous solution is obtained; S3, the PH value is adjusted to be 2-3, and acid filtrate and precipitates are obtained through filtering and precipitation; S4, after the precipitates are dried, an extract of the precipitates are obtained; S5, the PH value of the acid filtrate is adjusted to be 9-10, and alkaline filtrate is obtained; S6, the alkaline filtrate is extracted through dichloromethane, dichloromethane extraction liquid is obtained, and after concentration and drying, a dichloromethane extract-phase extract is obtained; S7, the alkaline filtrate is extracted through n-butanol, n-butanol extraction liquid is obtained, and after concentration and drying, an n-butanol extract-phase extract is obtained; and S8, sulfuric acid is added into the alkaline filtrate to adjust the PH value to be neutral, and an extract of a mother solution isobtained. The monoamine oxidase A inhibitor can be applied to prevention and treatment of related diseases with the excessively-high monoamine oxidase A activity.