A method for preparing a zinc sulfide phosphor having effective emission at low voltages. In the method for synthesizing a green-light emitting zinc sulfide based phosphor, ZnS:Cu, Al, which is applicable to a device operated with a low voltage, novel raw materials are employed. CuI is used as an activator, wherein monovalent copper (Cu1+) is complexed with ammonia and the ammonia complex is incorporated so as to uniformly distribute the activator in the mixture of raw materials. AlF3 is used as a co-activator, and, ZnI2 and NaI are added as iodide series fluxes. Thus, the color display characteristics are improved with a relatively high color purity and luminance.