Provided are a culture medium and a preparation method thereof and a method for cultivating cordyceps militaris by using the culture medium. The culture medium comprises a culture liquid and a substrate; the culture liquid includes a major element aqueous solution, a trace element aqueous solution, an organic matter aqueous solution, a salt aqueous solution and a glucose aqueous solution; the substrate is uncooked rice. The preparation method includes the steps: soaking the substrate with the culture liquid for 8-12 h at the temperature of 18-22 DEG C and sterilizing. The method for cultivating cordyceps militaris includes the steps: inoculating the culture medium with a cordyceps militaris strain, and cultivating in a darkroom; putting into a 24-h full-spectral irradiation cultivation room, and cultivating; scratching on the surface of the strain culture medium; allowing to stay in the full-spectral irradiation cultivation room with the temperature of 18-22 DEG C and the illumination intensity of 80-100 lux for 3-5 d; and when a bacterial vaccine grows to 0.2-0.4 cm, transferring the bacterial vaccine into a full-spectral cultivation room with the temperature of 18-22 DEG C and the illumination intensity of 190-210 lux for 23-27 d. With the use of the culture medium and the method provided by the invention, the growth cycle is short, the nutritional value is high, and the antibacterial ability is strong.