The invention belongs to the technical field of chemical production, and discloses a carbon molecular sieve and a preparation method thereof, the preparation method comprises the following steps: firstly, a mineral water bottle is pulverized and sieved by a 100 mesh to 150 mesh screen; an intercalation agent and an oxidizing agent are mixed evenly and expanded; the sample is further finely crushed, a binder, an adhesive additive and water are added for kneading, extrusion molding is performed, a molded matter is dried, carbonized and activated, and holes are regulated by carbon deposition to obtain the carbon molecular sieve. The preparation method has the advantages of wide source of raw materials, low cost, simple process, saving energy and reducing consumption (energy saving effect is that energy consumption is reduced by more than 50% compared with that of the traditional process), short production cycle, little pollution and high production efficiency, and is an ideal preparation method of the carbon molecular sieve. The preparation method also solves the problem of environmental pollution.