Abstract: The development of the passive oxide film on the primary loop during hot functional testing of PWR nuclear power plants is very important in the minimizing corrosion of the materials and corrosion product release. In this paper, based on the theoretical research and engineering practice, it is concluded  that the electrochemical reaction and the chemical reaction exist in the formation of the passive oxide film  during the hot functional testing, and the growth mechanism of the double-layer film is explained. The reasonableness of the analysis of passivation test by electrochemical measurement method is demonstrated. The functional relationship between passivation temperature and passivation film reaction rate is deduced. When the temperature increases, the reaction rate will increase and the passivation time shortens. The theoretical limit of passivation temperature should not be lower than 260℃.