During the course of cognitive aging, the structure and function of brain change at different levels. Brain imaging techniques have become important means for the neural mechanism research of cognitive aging since 1970s. Functional imaging studies have consistently found two “overactivation” models in regional brain area in the elderly:hemispheric asymmetry reduction in older adults (HAROLD) and posterior-anterior shift in aging (PASA). The theories and hypothesis relating to the two models, including compensation theory and dedifferentiation theory are reviewed, and the neural mechanism of cognitive aging is explained from different aspects in this paper.