The invention relates to green phosphor powder and a preparation method thereof; the chemical characteristic expression of the green phosphor powder is Gd(1-x-y)2O2S:Tbx, Dyy, wherein, Gd2O2S is the matrix and Tb3+ and Dy3+ are the activator. The phosphor powder is prepared by adding sulfur, dopant and fusing agent in a vacuum sintering way. The phosphor powder is synthesized directly in a vacuum low-temperature sintering way without reducing atmosphere; the sintering temperature is 500 to 1000 DEG C; and the sintering time is 1.5 to 4.5 hours. Phosphor powder with better performance can be prepared under lower temperature and shorter heat preserving time; and the prepared powder has smaller granularity and reveals better luminous performance. The phosphor powder can emit green fluorescence under the excitation of ultraviolet ray and X-ray; when the excitation wavelength is 254nm, the main emission wavelength is 545nm; the granularity of the prepared powder can reach tens of nanometers; simultaneously, the green fluorescence is emitted; and the prepared powder has high luminous efficiency and stable property.