Abstract: This paper takes the protection system cabinet in Qinshan Nuclear Power Plant as an example, based on the existing required response spectrum and test response spectrum of functional component inside cabinet and cabinet frame and enclosure, an easy CDFM（Conservative Deterministic Failure Margin） analysis method mentioned in EPRI NP-6041-SL technical report is adopted to calculate the seismic margin earthquake scale factor through parameters’ selection such as clipping factor, effective cabinet amplification factor, knockdown factor and so on to find seismic margin of existing protection system cabinet.