Insulin has recently been proved to be closely associated with central degenerative diseases such as Alzheimer's disease (AD) and may be a target for the treatment and improvement of memory and cognitive performance of AD patients. However, the blood-brain barrier prevents subcutaneously administered insulin from reaching the desired concentration in the brain. The nasal route is the easiest and most efficient way to avoid the first-pass effect of the liver and directly deliver drugs into the central nervous system. Clinical trials have demonstrated that intranasal insulin can improve the cognitive function of AD patients. This article reviews the advantages and disadvantages of different insulin delivery routes, the relationship between insulin and AD, and the efficacy of intranasal insulin in the treatment of AD.