ObjectiveTo explore the evolution principles of symptoms including deficiency， phlegm and blood stasis， and of the functional near-infrared spectroscopy （fNIRS） cerebral hemodynamic characteristics at various stages in patients of Alzheimer's disease. MethodsA total of 497 patients with complaint of memory loss were included， and were divided into subjective cognitive decline （SCD） group （198 participants）， mild cognitive impairment （MCI） group （228 participants） and dementia （AD） group （71 participants）. Neuropsychological evaluation， traditional Chinese medicine （TCM） syndrome investigation， and fNIRS data collection of prefrontal cortex were performed in each group. Descriptive statistics were used to analyze the distribution of TCM syndromes and the difference of TCM syndrome scores in each group； logistic regression was used to analyze the influence of TCM syndromes on the incidence of the patients； association rules were used to analyze the TCM syndromes of the patients； the hemodynamic characteristics of fNIRS in the prefrontal cortex of each group were compared. ResultsKidney essence deficiency syndrome was the dominant syndrome in all stages of AD. There were statistically significant differences in the distribution frequency of kidney essence deficiency， phlegm turbidity obstructing orifices， blood stasis obstructing collaterals， qi and blood deficiency， heat toxin in the interior， and fu-organ stagnation and turbidity retention syndromes among the three groups （P＜0.01）， and the scores of kidney essence deficiency syndrome among the three groups were statistically significant （P＜0.01）. Logistic regression analysis showed that kidney essence deficiency， and qi and blood deficiency syndromes were the main risk factors for the SCD group （P＜0.05）， phlegm turbidity obstructing orifices syndrome was the main risk factor for the MCI group （P＜0.05）， and heat toxin in the interior， and fu-organ stagnation and turbidity retention syndromes were the main risk factors for the AD group （P＜0.05）. The association rule analysis showed that the combination of kidney essence deficiency plus phlegm turbidity obstructing orifices had the highest support （33.33%） in the SCD group， and the combination of kidney essence deficiency plus blood stasis obstructing collaterals had the highest support （32.90% and 52.13%） in both the MCI and AD group. The prefrontal fNIRS results showed that the mean ∆HbO₂ concentration in the left dorsolateral prefrontal cortex （LDLPFC） decreased sequentially among the three groups （P＜0.05）， and the mean ∆HbO₂ concentration in the LDLPFC was negatively correlated with the MoCA score among the three groups （r = -0.142， P＜0.05）. Further analysis showed that the mean ∆HbO₂ concentration in the LDLPFC of patients with kidney essence deficiency syndrome were statistically significant differences among the three groups （P＜0.05）. ConclusionKidney deficiency is the basis of the pathogenesis of AD， and the key brain area damaged is the LDLPFC. Turbid pathogens such as phlegm and blood stasis are the pathological factors that aggravate the disease， and the syndromes of AD show the evolution law of deficiency and excess as “kidney deficiency→phlegm turbidity→blood stasis→turbid toxin”. The changes in prefrontal hemodynamics based on fNIRS are consistent with the changes in the characteristics of symptoms， which can be used to assess the degree of cognitive impairment in AD patients.