In order to reveal the change of cellulose structure and physical property caused by ultra micronization, cellulose power was treated through different concentrations of sodium hydroxide and then broken by using high pressure jet. The microstructure, size and physical properties of microfabrication were explored. After pretreatment with 10% NaOH solution (70 ℃, 1 h) and high pressure jet treatment, the diameter of the crude microfabrication decreases from 5.0~50.0 μm to 0.1~1.0 μm, and the length decreases from 20.0~200.0 μm to 1.0~10.0 μm. The peak value of cellulose particle size distribution curve was reduced from 158.0 μm to 10.0 μm. The specific surface area was improved by 3.0~ 5.5 times. The results indicated that the swelling of cellulose microfiber bundles after alkali pretreatment could reduce the mechanical properties of the cellulose, and its particle size decreased significantly by the spraying of high pressure jet. XRD results showed that there was no obvious change in the crystallinity and thermal stability after high pressure jet. However, the crystallinity of cellulose decreased 10% after alkalization. Alkali pretreatment improved the micronization degree of cellulose by high pressure jet.