High-fat diet (HFD) is considered a risk factor for age-related memory impairments such as Alzheimer's disease. However, how HFD affects memory formation remains unclear. In this study, we established a model of memory defects caused by HFD in Drosophila. Our results revealed that the HFD impaired intermediate-term memory (ITM), but not short-term memory (STM), produced by classical aversive olfactory conditioning, and decreased autophagic activity in the heads of the HFD-fed flies. Transient reduction in autophagic activity also impaired ITM, but not STM. Genetic enhancement of autophagic activity in neurons effectively restored ITM performance in the HFD-fed flies. Mechanistically, HFD impairs lysosomal function by downregulating the expression of lysosome-related genes, leading to impaired fusion of autophagosomes with lysosomes. These findings suggest that HFD impairs ITM by reducing autophagic activity and lysosomal dysfunction in the neurons.