The present invention provides active substances of Bifidobacterium lactis GKK2, a composition comprising the same, and a method of promoting longevity using the same. Specifically, the invention provides the composition for promoting longevity, which comprises bifidobacterium lactis, and the bifidobacterium lactis is preserved in China General Microbiological Culture Collection Center (CGMCC) with the preservation number of CGMCC No. 15205. The composition disclosed by the invention can be particularly used for increasing the expression quantity of the Cisd2 gene, reducing mitochondrial damage and delaying aging phenomena such as neurodegeneration and amyotrophy.