The invention relates to a modified compound kieselguhr adsorbing agent for treating industrial wastewater and a preparation method. According to the technical scheme provided by the invention, the method comprises the following steps: firstly, modifying a diatomite subjected to ore-dressing purification and calcinations by using an organic quaternary ammonium salt, and then uniformly mixing the modified diatomite, attapulgite and de-magnesium silicon residues of serpentine at mass percentage of (60%-75%):(5%-35%):(5%-35%), thereby preparing the modified compound kieselguhr adsorbing agent. In the modified compound kieselguhr adsorbing agent, three porous mineral materials, namely, the diatomite modified by quaternary ammonium salt, the attapulgite and de-magnesium silicon residues of serpentine can give play to individual strengths, can collaboratively match one another and can selectively absorb the organic and inorganic pollutants in different types and molecular dimensions. The modified compound kieselguhr adsorbing agent is fit for treating the industrial wastewater in complex pollutant components and higher concentration.