The invention relates to a preparation method of a special gas, in particular to a preparation method of an electric light source gas containing dibromomethane. A gravimetric method is adopted for preparation, a dibromomethane liquid sample is weighed quantitatively at room temperature, the temperature is increased to 70-80 DEG C and kept constant for 8-12 min, after the liquid sample is completely evaporated, balance gas is used for performing voltage transformation purging on the dibromomethane gas to a product steel cylinder, finally the steel cylinder is filled with the balance gas to the product pressure being P, mixing is performed, the actual concentration of the product is obtained through calculation, preparation is completed, and the balance gas is xenon or krypton. The method can solve the problem that the concentration of the electric light source gas does not reach the standard due to the fact that dibromomethane is easily stuck to a vessel wall, and remarkably improve the yield and product concentration uniformity of the electric light source gas containing dibromomethane.