A new method for preparing long afterglow phosphor and properties had been studied in the paper. In the new process the activated Al-Sr alloy powder hydrolyzes in pure water. The hydrolysates contain the mixture of Sr(OH)2.8H2O and Al(OH)3 in main, included a little Al(OH)3, AlOOH, SrCO3, and 3SrO.Al2O3. 6H2O. After heat treatment at 700°C for 1 hour, the mixture of SrO and α-Al2O3, γ-Al2O3 were formed. As a precursor, the hydrolysates after heat treatment had been used to prepare long after phosphor Eu, Dy co-doped SrAl2O4. The crystal structure of hydrolysates and Eu, Dy co-doped SrAl2O4 was analyzed by XRD. The luminescent properties of long afterglow material were studied. The Results showed the emission spectra of Eu and Dy co-doped SrAl2O4 lied near 520nm, resulted from the character transition of 4f5d-4f of Eu2+ ion. The method can improve afterglow properties dramatically. The initial luminescent intensity of long afterglow material reached 18cd/m2, and the duration time lasted 46 h.