The optical processes of alkali antimonide photocathodes are investigated. Based on the similarity of structure between the antifluoride and alkali antimonide

the possible transitions taking place in the Brillouin zone corresponding to the structure in the optical spectra are identified for Na

2

KSb（Cs）

Css

3

Sb and K

2

CsSb cathode. It is found that the optical transition of Na

2

KSb（Cs） is similar to that of crystalline film and Cs

3

Sb and K

2

CsSb are similar to amorphous film. The peak arising from the contribution of Δ and Σ direction in Na

2

KSb（Cs） isn’t observed in Cs

3

Sb and K

2

CsSb. We explain the transformation from nondirect transition to direct transition which takes place in alkali antimonides and draw a conclusion that nondirect transition in which momentum K conservation doesn’t play an important rule in optical transition due to disorder remains in the polycrystalline film with some degree of disorder. The multialkali effect is also explained.