In order to control the blockage of deep wells in Yingxi area，Qaidam Basin，reservoir rock characteristics and precipitate composition were studied by scanning electron microscope and ion titration technology. The blocking mechanism was also analyzed.The prevention technology was used to carry out field operation in seriously blocked well，which was injected deposition inhibiting composition through annularspace. The results showed that there were NaCl crystal particles in the pore，rock surfaces and fracture of reservoir. The blockage was mainly NaCl with cubic crystal structure. In well hole and swabbing process，NaCl crystal particles formed under the condition of changing reservoir thermodynamics，crystallization kinetics and hydrodynamic. Moreover，plugging in oil wells would be caused by crystallization deposition，as the enrichment of inorganic matter and a few organic impurities enhanced. The sediment inhibition rate of blockage was 90.0%—94.3% after prevention technology which was injected deposition inhibiting composition through annularspace. The load and shape of indicator diagram became smoother after treatment. The well washing cycle was extended for 44 days and the production stability of oil well was improved.