Abstract: It is a major theoretical proposition put forward by the Chinese Communists in the new era that the world is undergoing great changes unseen in a century. To deepen the study and understanding of the great changes in the world unseen in a century, it is necessary to carry out the explanation of the logic in Marxist theory, centering on ontology, epistemology and methodology. The first is to analyze, in combination with history and reality, the contemporary vision and development of vast changes unseen in the world in a century. The second is to clarify the stages of understanding and development of the unprecedented changes in the world on the basis of practice. The third is to master the correct way to deal with the unprecedented changes in the world under the guidance of a scientific worldview. Only in this way can it provide ideological guidance and practice for Chinese Communists to scientifically evaluate and correctly grasp their own external environment in the new era and new journey.