Based on the eight‑dimensional framework of Prim‑Lex theory, this paper redefines Alzheimer’s disease (AD) as “an irreversible phase transition of the brain‑body system from a global coherent state to a localized decoherent state.” The onset of AD is not a misfolding of a single protein but a collective phase transition of the eight‑dimensional system from a global coherent state to irreversible decoherence: decoherence of the NADH/NAD⁺ redox couple in brain energy metabolism (Prim‑Unity·Life Prim‑Fire) → loss of metabolic flexibility (Two Principles·Yin‑Yang polarization) → disruption of cross‑scale information transmission (Three Realms·GC⁴A) → abnormalities in the neuronal cell cycle (Four Phenomena·Four Colors disorder) → metabolic reprogramming of microglia and the vicious cycle of neuroinflammation (coupling imbalance of the Five Elements·Five Models) → collapse of the mitochondrial network and organelle spatial structure (decline in the Six Dimensions·Six Directions modulus) → disintegration of circadian rhythm and sleep‑wake cycles (desynchronization of the Seven Luminaires·Seven Rhythms) → entropy explosion of the immune‑neural‑glial information network (spike in Eight Trigrams·Eight Information entropy). This paper systematically elaborates the AD pathological manifestations, diagnostic methods, and targeted therapeutic strategies for each dimension, demonstrates the scientific validity of the eight‑dimensional comprehensive coherent‑state criterion as a unified benchmark for AD diagnosis, efficacy evaluation, and relapse prevention, and proposes a four‑tier clinical pathway for “life‑cycle AD prevention – screening – critical window intervention – systematic rehabilitation” based on the eight‑dimensional framework, thereby completing the scale bridging of Prim‑Lex theory from “planetary civilization governance” to “cognitive health governance.”