Objective: To study the effect and mechanism of the high-pressure jet mill on the stability of oat pulp, and to provide a theoretical reference for the processing of whole grain beverages. Methods: The oat pulp was processed with different pressures and high-pressure jet mills, and the morphology, instability index, particle size, and rheological properties of the storage period (30 d) were compared. Moreover the microscopic morphology (optical microscope, laser confocal scanning microscope), scanning electron microscope), soluble component content (soluble solids, soluble protein, soluble dietary fiber) were analyzed. Results: The high-pressure jet mill treatment gradually reduced the particle size, instability index, and apparent viscosity of oat pulp, and slowed down starch aging and particle aggregation. The high-pressure jet mill could homogenize protein and grease, destroy cell wall tissue fibers, dissolve more soluble substances, and create cavities in the particles, resulting in hydration capacity increase, and the improvment the system stability. Conclusion: The high-pressure jet mill technology can improve the storage stability of whole-grain products and prolong the shelf life.