The invention discloses a method for preparing nano mineral fiber, relating to the preparation of mineral fiber. The steps of the method are as follows: (1) purifying by a purifying agent inorganic acid, (2) stirring at a high speed of 2500-3000r/min and adding a coupling agent for coupling modification, (3) drying at the temperature of 60-150 DEG C, and (4) carrying out supersonic speed airflow deep defasciculation treatment for 2min-3min by an airflow mill to obtain nano mineral fiber products with the average diameter of 60-90nm and the length of 2-8 mu m, wherein the airflow pressure is 0.6-1.0 MPa, and the stepped gear rotated speed is 2500-3000r/min. The method of the invention is simple and feasible, and has stable process, low equipment investment and small environmental load; thediameter of prepared nano mineral fiber has nano grade, and the length of fiber can be effectively maintained, thereby completely meeting the requirement of preparing organic and inorganic nano composite materials with high performance.