The binding energy,energy band structure and optical properties of the oxygen physical adsorption on semiconducting single_wall carbon nanotube are studied by the density functional theory. The most stable adsorption site is the one in which the oxygen molecule lies above the center of the carbon ring. It is showed th at the oxygen physical adsorption strongly affects the transport properties and the optical absorption spectra of carbon nanotubes. And the phenomenon of oxygen photoinduced desorption has been explained theoretically.