Design diversity was identified as an effective method to decrease CCF rate of SIS. However, the SIS designed based on design diversity cannot be quantitatively assessed using the current reliability assessment techniques. Therefore, a novel method for researching the effect of diversity on CCF mitigation from the component degradation state point of view was worked factors （IFs） influencing significantly the failure behavior of the out. Firstly, four categories of impact component were defined. Corresponding methods of mapping diversity into component degradation valve （CDV） were proposed according to the specific characteristics of each IF category. Then CCF analysis was performed based on Alpha factor model and weighted impact vectors which can be calculated from CDVs and thus the relationship between diversi- ty and CCF rate was studied. Finally, an application example was provided that illustrated how the proposed method performed for a chemical reactor safety system. The results show that the CCF rate corre- lates with diversity between redundant components, and that the reliability of SIS can be improved by using design diversity technique.