A kind of natural concentrated dietary fiber and preparation method thereof, described natural concentrated dietary fiber is that the pine bark that is rich in dietary fiber is crushed and sieved, soaked in water, drains waste water after soaking, and then transfers the soaked pine bark To the reactor, add water to obtain the pine bark mixed liquid, carry out ultrasonic-assisted enzymolysis treatment, drain the liquid after enzymolysis, then insert the enzymolysis material into Lactobacillus plantarum HKB‑38 for fermentation, and ferment the fermented The material is subjected to high temperature and high pressure heat treatment, cooled and dried to obtain natural concentrated dietary fiber. The invention also includes a preparation method of natural concentrated dietary fiber. The invention selects pine bark with low price and abundant resources as the production raw material, greatly reduces the production cost, makes full use of resources and creates high added value. The invention significantly improves the purity of the dietary fiber and the swelling and water holding capacity of the fiber product. Experiments have proved that in the concentrated dietary fiber of the present invention, the crude fiber content is ≥ 60%, the neutral detergent fiber (NDF) content is ≥ 86%, the acid detergent fiber (ADF) content is ≥ 70%, the product swelling property is ≥ 5.0mL / g, and the water holding capacity ≥5.0g / g.