Cellulose II nanocrystal particles have a crystallinity ≥80%, a number-average molecular weight ranging from 1200 to 2500, and a molecular weight distribution coefficient Mw/Mn≤1.30. The cellulose II nanocrystal particles can be prepared by: subjecting a cellulose raw material to an amorphization reconstitution and then to a crystallization acidolysis. The crystallization acidolysis may be carried out under a low concentration acidic condition. The method enables high efficient and clean production and quality control of cellulose nanocrystal materials.