Abstract: As the interface between the primary and secondary circuits in the steam generator（SG）, the heat transfer pipe plays an important role in the long-term safe and stable operation of nuclear power plant. In order to better evaluate the structural integrity of the SG in long-term operation, it is necessary to carry out the SG hidden impurity return test in combination with the overhaul and shutdown of reactor, and evaluate the local hideout return condition of the SG secondary side. As the design of the SG of the AP1000 power plant differs from that of other second generation nuclear power plants, the evaluation method of the SG hideout return test for the AP1000 nuclear power plant are different from that for other traditional power plants. Based on the design characteristics of the SG of AP1000, this paper developed a set of test methods and evaluation procedures for the specific SG hidden impurity return in AP1000, and applied them during the overhaul of Units 1 ＆ 2 of Sanmen Nuclear Power Plant. This study aimed at providing guarantee for the water quality optimization of the secondary circuit and the continuous optimal hydrochemical environment of the SG. The findings of this study can be used as a reference for the hydrochemical control during the design, commissioning, startup and normal operation of new units.