With the long term immersion test of C-ring specimens loaded by two levels of load value, the caustic stress corrosion cracking resistance of three kinds of steam generator tubing alloy 690 in 50% NaOH at 325 ℃was evaluated. The stress value after loading was measured by XRD method and the oxide film after immersion was carefully analyzed. The results show that: (1) The maximum stress value of C-ring specimens is released to a certain extent after loading slowly by screw, and the release amount of load value of the domestic tubes is larger than that of the imported tubes; (2) The domestic tubes and the imported tubes all have good caustic-SCC resistance in 50% NaOH medium at 325℃, and the oxide film characteristic of the domestic tubing A are closer to that of the imported tubing C; (3) The good resistance of caustic stress corrosion of tubing alloy 690 in high concentrated alkaline medium at high temperature is related to the double-layer structured oxide film and the continuous carbide structure along grain boundary.