The invention relates to silicate blue light fluorescent powder having an expression formula of Gd4.67Si3O13:xBi<3+>, wherein doped Bi<3+> is an active ion, and x ranges from 0.01 to 0.2. According to the fluorescent powder, a small amount of Bi<3+> ions are introduced into a Gd4.67Si3O1 matrix, and under irradiation of ultraviolet light with the peak value being about 370nm, and bandwidth emission close to 450nm can be generated. The preparation method of the blue light fluorescent powder comprises the following steps: respectively weighing elementary substances, oxides, halides or corresponding salts of Gd, Si and Bi according to amounts of substances in the expression formula; grinding and fully mixing in an agate mortar, putting in a corundum crucible and covering, and heating in a high-temperature furnace to 1200-1400 DEG C, holding the temperature at 1200-1400 DEG C constantly for 2-8 hours; and then, naturally cooling to room temperature, taking out and smashing to obtain the adjustable blue light emission fluorescent powder.