Abstract: A method for constructing a micro-nano porous organic acid pretreatment layer on the metal surface and its application are provided. A pretreatment reagent used is prepared by chemically grafting organic acid with hydroxylation-rich polymer. After immersed in the pretreatment reagent for a period of time, the metal substrates are then dried and cured, and the organic acid pretreatment layer with a micro-nano porous structure and excellent adhesion is successfully formed. The present invention provides the pretreatment reagent and pretreatment process of the metal surface coating protection technology. The solvent is only pure water and realizes no organic solvation. The pretreatment layer has excellent adhesion with the metal substrate, rough and porous structure, and can be closely interlocked with the subsequent coating layer, which greatly enhances the anticorrosion performance of the whole coating system.