The invention provides a preparation method of alpha-Sialon fluorescent powder which is the fluorescent powder for the blue LED and UV LED. The alpha-Sialon fluorescent powder can be expressed by the general formula (M1)x(M2)ySi12-(m+n)Alm+nOnN16-n, wherein M1 is one or more elements of Li, Mg, Ca, Y, Sr and Gd, M2 is one or more elements of Eu, Ce, Pr, Tb, Yb, Dy and Er, 0.3<=x+y<=1.8, 0<=y<=0.7, 0.5<=m<=4.0, 0<=n<=2.5, and x+y=m/(average valence of M1 and M2). The synthesis process of the alpha-Sialon fluorescent powder comprises the following steps: firstly performing ball milling to raw materials, drying, then synthesizing through propagating technology, finally grinding and sorting or directly sorting to obtain the alpha-Sialon fluorescent powder. The preparation method of the invention has simple devices, high efficiency and low cost and the industrial production is easy to realize.