Doppler broadening spectroscopy in the positron annihilation technique (DBS-PA) has been employed to investigate the defect properties in argon-damaged tungsten exposed to low-energy and high flux deuterium plasma. Argon ion irradiations with energy 500 keV are performed for tungsten samples with various levels of damage. The remarkable increment of the S parameter in DBS-PA indicates the introduction of vacancy-type defects in argon irradiated tungsten. An increase of ion fluence results in a continuous increase of the S parameter until saturation. Unexpectedly, a much higher fluence leads to a decrease of the S parameter in the near surface, and the (S,W) slope changes greatly. This should be associated with the formation of argon-vacancy complexes in the near surface produced by the excessive implanted argon ions. With deuterium plasma exposure, a significant decrease of the S parameter occurs in the pre-irradiated tungsten, suggesting the sharp reduction of the number and density of the vacancy-type defects. The thermal desorption spectroscopy results demonstrate that the argon-damaged tungsten, compared to the pristine one, exhibits an enhanced low-temperature desorption peak and an additional and broad high-temperature desorption peak, which indicates that deuterium atoms are trapped in both low-energy and high-energy sites. All these observations directly indicate the deuterium occupation of irradiation-induced vacancy defects in damaged tungsten, which is responsible for the remarkable increase of the deuterium retention in comparison with the pristine one.