Abstract: The nuclear power plant containment is an important physical barrier to ensure the safe operation of nuclear power units. The Containment Tightness Test (CTT) is an important means to verify the strength and tightness of the containment structure. Before the nuclear power plant is built and put into use, the CTT is used to verify whether the construction quality and structural performance of the containment meet the design requirements. Due to the complexity of the prestressed containment structure, it is difficult to accurately calculate and predict the structural response of the containment under the pressure test. Taking a third generation nuclear power plant as the research object, a refined three-dimensional finite element model of the containment was established based on ANSYS. The overall deformation and strain of the structure were continuously monitored during the prestressed tension and CTT of the containment. The initial analysis parameters of the finite element model were modified through the coincidence analysis of the measured and calculated values of concrete strain during batch tensioning of prestressed tendons. Based on the modified finite element model, the structural deformation of the containment under test pressure were calculated and predicted before the CTT. The monitoring measured data during the CTT showed that the structural performance of the containment under test pressure can be accurately calculated and predicted by the above finite element model modification method.