A malic acid and KMnO4-based combined and modified cow dung biogas residue hydrochar preparation method, comprising: mixing a cow dung biogas residue with malic acid, and performing ultrasonic treatment to obtain a malic acid modified cow dung biogas residue; performing a hydrothermal reaction with KMnO4 in a high-temperature high-pressure reactor to obtain a combined and modified cow dung biogas residue hydrochar material.