The invention discloses a process for preparing a brucite inorganic flame retardant, comprising the following steps of 1) crushing the brucite raw ore: respectively adopting a jaw crusher and a hammer crusher to carry out two-grade crushing treatment to the brucite raw ore so that the particle size of the crushed brucite is less than 3mm; 2) sending the crushed brucite into a superfine ball mill for grinding and then sucking the crushed brucite into a superfine classifier by an induced draft fan at the material outlet of the superfine ball mill; and 3) carrying out circular grading treatment of the grinded brucite in the superfine classifier for three times; subsequently sucking the brucite after primary grading into the superfine classifier for the secondary grading; subsequently sucking the brucite after the secondary grading into the superfine classifier to carry out the third grading; releasing the selected slag to the environment; leading the brucite after grading by three times to enter a finished product collector; and the average particle size of the brucite after grading by three times is less than 5 mu m. The average particle size of the brucite prepared by the process is less than 5 mu m; and the brucite treated by surface modification has excellent compatibility with macromolecule materials and greatly improves the flame-retarding performance of the product.