Astrocytes are fundamental for maintaining brain homeostasis and are commonly involved in the progression of neurodegenerative diseases including Alzheimer's disease (AD). In response to injury or toxic material, astrocytes undergo activation that results in hypertrophy and process ramification. Although numerous studies have shown that reactive astrocytes are intimately related to the pathogenesis of AD, their characteristic features including morphological and molecular alterations that occur during different stages of AD progression remain to be elucidated. Here, we crossed astrocyte‐specific reporter mice hGFAP‐CreERT2;Rosa‐tdTomato with APP/PS1 mice, and then used genetic tracing to characterize the morphological profiles and expression of molecular biomarkers associated with progressive β‐amyloid deposits in the cortical region of AD mice. Expression of glutamine synthetase (GS) was lower in cortical reactive astrocytes, in contrast to the higher expression of glial fibrillary acidic protein, of APP/PS1 mice and AD patients relative to that in cortical astrocytes of wild‐type mice and age‐matched controls, respectively. GS activity was also decreased obviously in the cortex of APP/PS1 mice at 6 and 12 months of age relative to that in the wild‐type mice of the same ages. Furthermore, cortical reactive astrocytes in APP/PS1 mice and AD patients did not undergo proliferation. Finally, based on RNA‐sequencing analysis, we identified differentially expressed transcripts of signal transduction molecules involved in early induction of reactive astrocytes in the cortex of APP/PS1 mice. These findings provide a morphological and molecular basis with which to understand the function and mechanism of reactive astrocytes in the progression of AD.