The electrochemical behaviors of Alloy 690 was studied in simulated PWR secondary loop high temperature and high pressure alkaline water environments controlling pH (pH_@25℃=9.5) with ammonia-water by measuring open circuit potentials (OCP), electrochemical impedance spectroscopies (EIS), potentiodynamic (PD) polarization curves and thermomechanical analysis. The results showed that: After immersing in ammonia-water at the given temperature (50, 100, 150, 200, 250 or 285℃) for 20h, the OCPs of Alloy 690 have reached a rather steady state and the double-layer oxide films have formed on the Alloy 690. With the increasing temperature, the resistances of outer oxide layer firstly decrease and then increase, while the resistances of the inner oxide layer and the protection of the whole oxide films decrease. In addition, the solution pH_@T decreases with the increasing temperature. All of the above causes the decline of OCPs, corrosion current densities and passive current densities from the PD curves.