Based on the high temperature and high pressure corrosion tests, the corrosion behavior of titanium alloy OCTG (oil country tubular goods) in high pH value completion fluid was studied by in-situelectrochemical measurement, the stability of titanium alloy passivation film was investigated by molecular dynamic simulation of thermodynamic, and the reaction enthalpy was calculated by using first-principles method. The results show the corrosion of TC4 alloy in high pH value phosphate completion fluid is very serious, and the uniform corrosion rate is as high as 0.4429 mm/a.The corrosion reaction of TC4 alloy in high pH value phosphate completion fluid is controlled by anodic reaction process.With the increase of temperature, the corrosion potential, the film resistance of corrosion product scale and the polarization resistance decrease significantly, the thermodynamic driving force of corrosion reaction increases, while the dynamic resistance decreases, and the corrosion current density of TC4 alloy increases significantly. In TiO₂ passivation film, the titanium-oxygen bonds of TiO₂ is tending to break due to the immersion of alkaline solution. Furthermore, the interfacial bonding energy between TiO₂ and aqueous solution increases gradually with the increasing of temperature and alkalinity, so the thermodynamic stability of passivation film decreases. In alkaline potassium pyrophosphate solution, TiO₂ passivation film can react with K₂HPO₄ and K₃PO₄ to form porous KTiOPO₄ corrosion product scale, but the thermodynamic tendency of the reaction between TiO₂ and K₃PO₄ is more obvious.