Many former researches discovered that SrO/Al

2

O

3

ratio alteration in a bigger range would change the crystal phase composition of strontium aluminate doped by rare earth ions

and make crystals present different photoluminescence and persistent afterglow properties. However

the researches about the effect of SrO/Al

2

O

3

ratio in a small range on luminescence of phosphor dont systemically carry out up to now. In this paper

phosphor of Sr

2

AlO

4

:Eu

Dy

which has different SrO/Al

2

O

3

ratio

is synthesized by solid-state reaction. The effect of SrO/Al

2

O

3

ratio changed in a small range on phosphorescence of Sr

2

AlO

4

:Eu

Dy is studied. Experimental results show that the little alteration of SrO/Al

2

O

3

ratio cant cause the change of crystal phase composition of Sr

2

AlO

4

:Eu

Dy phosphor

but plays an important role in their photoluminescence and afterglow decay properties. The Al-rich phosphor has much better photoluminescence and longer afterglow life than that of Sr-rich phosphor. Emission spectra of the Sr

2

AlO

4

:Eu

Dy samples with different SrO/Al

2

O

3

ratio tell that emission intensity increases with the decrease of SrO/Al

2

O

3

ratio. Excitation spectra of the Sr

2

AlO

4

:Eu

Dy samples show that excitation intensity also increases with the decrease of SrO/Al

2

O

3

ratio and excitation spectra have big change at the range of 220～250 nm with the changes of SrO/Al

2

O

3

ratio. The appearance of new and intensive excitation peak at 220～250 nm when reducing SrO/Al

2

O

3

ratio

suggests that new electron energy level is formed and new electron transition takes place after excitation. From afterglow decay results

it can be found that Sr-rich sample shows lower luminance and shorter persistent time. In order to further investigate the root reasons for above phenomena

thermal stimulated luminescence (TSL) spectra are measured and the results indicate that changing SrO/Al

2

O

3

ratio will result in the changes of trap depth and trap density in the Sr

2

AlO

4

crystal lattice: the more Al

2

O

3

the bigger trap density and the lower trap depth. Based on above results and analysis of photoluminescence

afterglow decay and TSL

it can be concluded that in Sr-rich Sr

2

AlO

4

crystal the chances for Eu

2+

Dy

3+

to replace the positions of Sr

2+

is so slim that the density of luminescent central and traps in crystal lattice is very low. It is this reason that makes Sr-rich samples perform worse photoluminescence and afterglow decay properties.