Four novel acoranes (1–4), a new cadinane (5), and a new guaiane (6), together with six known sesquiterpenoids, were isolated from the rhizomes of Acorus tatarinowii. Their structures were established on the basis of extensive spectroscopic analysis. The absolute configurations were determined by single crystal X-ray diffraction, electronic circular dichroism (ECD), CD exciton chirality, and an in situ dimolybdenum CD method. The anti-Alzheimer effects of all isolates were evaluated by acetylcholinesterase (AChE) and β-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibition assays.