Energy security plays a critical role in national security. Researching into energy security requires to apply objective and reasonable indices to quantitatively evaluate the level, trend and causes of energy security. In this book, we establish the evaluation index system of China's energy security from four dimensions of the causes of energy security and determine the weights of the indices with entropy method to quantitatively measure and forecast China's energy security level from 2005 to 2022 based on the improved TOPSISI distance function model. Furthermore, we carry out grey relational analysis to investigate the factor with the highest degree of correlation with China's energy security and put forward suggestions for policy-making according to the above analysis.