The invention discloses green fluorescent powder and a preparation method thereof and belongs to the technical field of light-emitting materials.The general chemical formula of the green fluorescent powder is Li1-x-yRxCeyAlMgx+ySi5-x-yO12, wherein x is greater than 0 and less than 1, y is greater than 0 and less than 1, (x+y) is greater than 0 and less than 1, and R is one or more of La, Y and Lu.The preparation method comprises the steps of mixing a Li precursor, an R precursor, a Ce precursor, an Al precursor, a Mg precursor and a Si precursor, and conducting a high temperature solid phase reaction, so that the green fluorescent powder is obtained.The prepared green fluorescent powder has brand new chemical constitution, the synthesis temperature is relatively low, raw materials are low in price, Ce3+ serves as an activating agent, the fluorescent powder can be excited by ultraviolet light and purple-blue light to emit green light, thereby the fluorescent powder can convert the ultraviolet light and the purple-blue light into the green light, and therefore the fluorescent powder is applied to an LED for converting yellow fluorescent powder into white light and improves color rendering of the LED.