Abstract: Finite element method is applied to evaluate the isolation effect and classified stress strength of the HVAC and its hanger system with static stress analysis, modal analysis and seismic spectrum analysis, in which the shell element, space beam element, 3D isoparametric element, spring damper and lump mass element are adopted to build the exact 3D finite element (FE) model. The outcome indicates that the spectrum analysis results are nearly propotional to the load value as to the classified evaluation to shell and linear type component basing on ASME AG 1 criteria, whereas the damp hardly has effects on it except for restraining sympathy vibration. In order to make the conclusion convinced, it is necessary to ensure the sum of the mode participation mass beyond 90%. The proposed method is of reference value for the vibration isolation design and seismic strength evaluation for the HVAC in the nuclear power plant.