The invention relates to a nano diatomite treating agent, comprising 1-5% of nano-silica-base oxide, 15-0% of diatomite, 3-10% of bead alkali, 25-30% of anhydrous sodium metasilicate, 10-25% of sodiumsulfate, 6-8% of sodium dodecylbenzene sulfonate acid, and the balance sodium carbonate. The preparation method comprises the following steps: dispersing the nano-silica-base oxide in a high shear dispersion machine, transferring into a homogenizing stirrer, homogenizing while stirring, adding anhydrous sodium metasilicate and sodium sulfate in turn, adding the bead alkali, the diatomite, the sodium dodecylbenzene sulfonate acid and the sodium carbonate, homogenizing, stirring, and screening to obtain the powder nano diatomite treating agent or pelleting to prepare into graininess. The nano diatomite treating agent has the advantages of being non-toxic, no harmful, no polluted ecological protection green product with no harmful chemical elements, simplifying technological process, being easy to operate, outputting uniform and stable quality, improving production and use environment, no changing of the original equipment of factories, saving energy, reducing comprehensive production cost, prolonging the service life of the equipment and improving quality level of products.