In the present work, adsorptions of Li,Na,K,Cs on GaAs(llO) surface are studied. Both the ideal and the relaxed surface are considered. The calculations are carried out by using the charge self-consistent EHT method with a cluster model. The results indicate that the surface atoms tend to the ideal configuration after adsorption, and the alkali atom lies at the symmetric plane perpendicular to the surface and crossing the surface Ga atoms along the [001] direction. The Fermilevel is located about 0.7eV above the valence band maximum, which is contributed by the interaction between the surface Ga atoms and the alkali atom, while in the valence band, the alkali atom is bonded mostly with the surface As atoms.