Abstract: ：[Objectives] To study the sealing performance of lifting holes applied to nuclear power plants under accidental working conditions. [Methods] n experimental setup was built to achieve quantitative measurement of steam micro-leakage. In this device, saturated steam at 0.66 MPa was charged into a pressurized chamber to simulate accident conditions, and the leaking steam was collected through a collection chamber with insulation layer without condensation. The changes in temperature, humidity, and pressure in the collection chamber are monitored in real time to estimate the leakage rate of steam. [Results] The experimental results show that the steam leakage rates of the lifting hole models with 1.5 mm and 3.0 mm composite gaskets were 13.93 g/24h and 11.91 g/24h, respectively, both meeting the low leakage design requirements, with the 3.0 mm gasket providing the better sealing performance. Analysis of the dry air and steam measurements with the same gasket and at the same pressure showed that the sealing coefficients nd4 obtained for both were very close, verifying the accuracy of the experimental device for steam microleakage measurements. [Conclusions] The adopted steam microleakage measurement method can be extended to various sealing structures or pressure systems, and provides a new measurement idea for evaluating the sealability of the stack chamber under accident conditions.