The invention provides a dustproof grease composition used for an optical instrument and a preparation method of the composition. The dustproof grease composition comprises the following components inparts by weight: 67-95.2 parts of base oil, 3-15 parts of a thickening agent, 0.5-5 parts of a structure improving agent, 1-8 parts of a solid lubricant, 0.1-2 parts of a metal deactivator, 0.1-2 parts of an antioxidant, and 0.1-1 part of an antirust agent. The invention also provides a preparation method of the dustproof grease composition used for the optical instrument. The method provided bythe invention adopts fumed silica to thicken dimethyl silicone oil, and the structure improving agent, the solid lubricant, the metal deactivator, the antioxidant and the antirust agent are used to prepare the dustproof grease composition used for the optical instrument; and the prepared composition has excellent high and low temperature performance, colloidal stability, thermal oxidation stability, anti-wear and extreme pressure performance, sealing performance, adhesion performance and protection performance, and is suitable for long-term dust prevention and lubrication of rotating parts such as screw threads of various military and civilian optical instruments (sighting telescopes, periscopes and the like).